Bacterial Genetics



 It is the science of heredity.


 Process by which all living things produce offspring like them self.


 A segment of DNA or a segment of nucleotides in DNA that code for a functional product. All genes within an organism comprise that organism’s genome.



It is a large nucleic acid molecule found principally in the chromosomes of the nucleus of a cell, that is the carrier of genetic information.



It is a nucleic acid slightly different in chemical composition than DNA, rarely exists as a double stranded molecule, differentiate in to three major types (messenger RNA [mRNA], transfer RNA [tRNA] and ribosomal RNA [rRNA]), that play an important role in gene expression.


Non-chromosomal Elements of the Genes


Not all genes are confined to the chromosomes many genes are also located on plasmids and transposable elements.

Both of these are able to replicate and encode information for the production of various cellular products. Although considered part of the bacterial genome , they are not as stable as the chromosome and may be lost during cellular replication, often without severe detrimental effects on the cell.


1. Transposable Elements:

They are pieces of DNA that move from one genetic element to another, from plasmid to chromosome or vice versa , unlike plasmids they do not exist as separate entities within the bacterial cell because they must either be incorporated into a plasmid or the chromosome

Two types of transposable elements:

 i. Insertion Sequences ( IS):

Contain genes that simply encode for information required for movement  among plasmids and chromosomes.

ii. Transposon:

Contain genes for movement as well as genes encode for other features such as drug resistance.

Both (IS and Transposon) coexist with chromosomes in the cells of many bacterial species and play a key role  in the exchange of genetic material.


2. Plasmid:

It is a small double stranded, self replicating , closed , circular DNA; the number of plasmids per bacterial cell varies extensively and each plasmid is composed of several genes some genes encode products that mediate plasmid replication and transfer between bacterial cells (conjugative plasmids). Unlike most chromosomal genes, plasmid genes do not encode for products essential for viability.

Plasmid, in whole or in part may also become incorporated in the chromosome.

From the medical point of view, plasmids confer most of the antibiotic-resistance properties of bacteria, several pathogen virulence factors and system for production of some antibiotics.


Classification of Plasmids:

 According to the genetic content of  plasmids, they are classified in to :

I. Colicinogenic Plasmid:

plasmid encodes small protein (colicin) that kills a variety of bacteria related to the producing organism. Each plasmid also encodes an immunity protein , so that the cell producing the colicin does not commit suicide.


II. Virulence Plasmid:

Encode various proteins involved in the pathogenicity process of the bacteria for e.g. production of toxins ( Tetanis, Anthrax , and Endotoxins ) . Some virulence plasmid encodes protein responsible for colonization for e.g. factor that allow adherence of bacteria to cells in the mucous tissue.


III. Dissimilation Plasmid:

Which have gene coding for enzymes that catalyze the catabolism of certain unusual sugars and hydrocarbons. some sp. of Pseudomonas can actually use such substances as toluene, camphor and high molecular weight hydrocarbons of petroleum as primary carbon and energy sources.


IV. F (fertility) Factor Plasmid:

They contain genes that codes for factors essential for conjugation (gene exchange) for e.g. F factor in E. coli, which is responsible for the production of, sex pili .


V. Drug - Resistance ( R ) Plasmid:

Carry antibiotic resistance genes usually with several genes encodes for different type of antibiotic resistance carried by a single R plasmid. All the drug resistance gene clustered on one region called the r determinant  while the gene involved in conjugational transfer located at the other region called Resistance Transfer Factor      (RTF), the r determinant or (r-gene) is a complex  collection of mobile genetic elements ( insertion elements and transposons) that enable the spread of the encoded antibiotic resistance properties throughout the bacterial population.











Many plasmids isolated from nature fit one or another of these categories in a simple way, but others do not. Certain large conjugative plasmids  encode multiple colicins along with antibiotic resistance and virulence factors.


Genetic Exchange ( Genetic Diversity )

Genetic change in bacteria is accomplished by three basic mechanisms:

·   Genetic recombination.

·   Genetic exchange.

·   Mutation.


1. Genetic Recombination:

In these process some segment of DNA that originated from one bacterial cell ( i.e. donor) enters a second bacterial cell (i.e. recipient ) and is exchanged with a DNA segment of the recipient ‘s genome, this is also called homologous recombination because the pieces of DNA that are exchanged usually have extensive homologous or similarities in their nucleotide sequences. After recombination the recipient DNA consist of one original, unchanged strand and the second from the donor DNA fragment that has been recombined. Recombination  occurs frequently  in many bacterial species including most of the clinically important species. (Figure A).


2. Genetic Exchange:

The opportunity of an organism for undergoing recombination depends on the acquisition of foreign DNA from a donor cell . there are three mechanisms by which bacteria physically exchange DNA including :


A. Transformation :

Transformation involves recipient cell uptake of free DNA released into the environment when a bacterial cell ( i.e. donor ) dies and undergoes lysis . This DNA , which had comprised the dead cell’s genome, exists as fragments in the environment. Once the donor DNA, usually as a singular strand , inter  the recipient cell, recombination with  the recipient’s homologous DNA can occur.

The mixing of DNA between bacteria via transformation and recombination plays a major role in the development of antibiotic resistance and in the dissemination of genes that encode factors essential to an organism’s ability to cause disease. Additionally, transformation is not limited to organisms of the same species , allowing important characteristics to be disseminated to greater variety of medically important bacteria. (Figure B).


B. Transduction:

 This method of gene exchange is mediated by viruses that infect bacteria (i.e. Bacteriophages). In their “life cycle” these viruses integrate their DNA into the bacterial cell’s chromosome, where viral DNA replication and expression is directed . when the production of viral products is completed. Viral DNA is excised (cut)  from the bacterial chromosome and packaged within protein coats. The viruses are then released when the infected bacterial cell lyses. in transduction the virus not only packages its own DNA but may also package a portion of the donor bacterium’s DNA.

The bacterial DNA may be randomly incorporated with viral DNA ( generalized transduction) or only bacterial DNA that was adjacent to viral DNA in the bacterial chromosome is packaged (specialized transduction)  in any case, when the viruses infect another bacterial cell they release their DNA contents, which may include bacterial donor DNA. Therefore, the newly infected cell is the recipient of donor DNA  introduced by the infecting bacteriophage and recombination between DNA from two different cells may occur. (Figure C).


C. Conjugation :

This process occur between two living cells , involves cell-to-cell contact, in  which the donor cell that contain F factor plasmid (F+) produce sex pili that attached to F- cell (which contain receptor sites that bind pili) that is called intracellular bridge .

Plasmid transfer: During conjugation the F factor containing plasmid first undergo replication so that the donor retains a copy of the plasmid that is being transferred to the recipient, and the new copy is transferred across the intracellular bridge from an F+ to F- cell. (Figure E)

Chromosomal transfer: Some times in the F+ cell the F factor integrated in to their chromosome and called High frequency of recombination (Hfr) cells. During conjugation between an Hfr and F- cell, the chromosome of the Hfr cell replicates and the new copy of the chromosome is transferred to the recipient cell , only the Hfr cells can transfer their chromosome. The amount of DNA transferred depends on how long the cells are able to maintain contact, but usually only portions of the chromosome are transferred .

In any case , the newly introduced DNA is then available to recombine with the recipient’s chromosome. (Figure D).



3. Mutation: ( Heritable Alteration in a Gene)

It is a change in the base sequence of DNA leading to the production of a protein that has an altered amino acid sequence.








Types of mutation

I. Point mutation or base substitution:

It is the simplest type of mutation in which a nucleotide in DNA is replaced with different nucleotide, it may or  may not result in change in the amino acid sequence.  if the mutation result in no change in the amino acid sequence it is called silent mutation .









II. Addition and Deletion :

Here one or few base pairs are deleted or added to DNA, this can shift the translational reading frame this called Frame shift mutation which is almost always result in an inactive protein product for the mutated gene.









III. Transposable - Element Mutagenesis:

Usually the Transposon is present in a non-essential region of the DNA, when it inserted into an essential region it will disrupt its function, because it will interfere with transcription of the gene into which it is inserted.






Classification of Mutation:

Mutation can be classified in a variety of ways the most usable system is the classification according to the mutational events

 They are classified into:

·   Spontaneous Mutations:

Are mutations that occur without known intervention of mutation -causing agents, as that occurs during DNA replication .

·   Induced Mutation:

In which agents in the environment such as certain chemicals and radiation (mutagens) cause mutation .eg Nitrous acid, Nitrogen mustard, Ultraviolet irradiation and Ionizing radiation .


Virulence factors


Several factors determine the readiness with which infectious disease occur in  an individual.


P = NV/R


Where p          Probability that disease will occur.

           N          No of microorganism in infecting dose.

           V          Virulence of microorganism.

           R          Host defense.



Virulence:  is the capacity of a given strain to produce disease.


    Virulence are attributed to two factors


  1. Toxogenicity: the ability to produce toxins
  2. Invasiveness: the ability to attach and multiply in host tissue.




 Toxins are normal cellular compounds that damage host tissue, they are Endotoxins or Exotoxins






Bacterial source

Mostly G +ve bacteria

Almost G-ve bacteria

Relation to M.O

Metabolic product of living cells.

Present in cell wall & only released when death of cell.


Protein or short peptide

Lipopolysaccharide complex.

Heat stability

Unstable destroy at 60 – 80 C.

Stable even in autoclaving (120 C for 1 hr).





Can converted to toxoids and neutralized by antitoxins

Can not converted to toxoids and not easily neutralized by antitoxins

Effect on body

Specific for a particular  host tissue

General (fever, weakness..)

Lethal dose



Representive disease

Gas gangrene, tetanus, botulism, diphtheria.

Bacillary dysentery.








It is the ability of microorganism to adhere to host tissue and as a result of microbial metabolism and multiplication cause structural damage to cell and interfere with metabolic reactions.


1.     Capsule:

The presence of capsules is associated with virulence of certain bacterial strain

Virulent forms of Streptococcus pneumoniae, Klebsiella pneumonia, Bacillus anthracis and Haemophilus influenzae posses capsules while non capsulated forms are avirulent. Capsules posses some chemical and physical properties that make them resist to phagocytosis.


Component of the cell wall:

       The cell wall of certain bacteria contain chemical substances that contribute to invasiveness.


2.     Lipoteichoic acid:

For an organism (e.g. Streptococcus pyogen) to infect, it must adhere to the host tissue first. It has been shown that adherence to oral epithelial cells is mediated by the lipoteichoic acid of Streptococcus pyogen.


3.     M protein :

 It is heat – acid resistant cell surface protein produced by certain Streptococcal sp. that will help the bacterium resist phagocytosis by WBC.


4.     Protein A:

Found in Staphylococcus aureus cell wall linked to the peptidoglycan, serve as antiphagocytic, anticomplementary and elicits hypersensitivity reaction.



The invasiveness of some bacteria is aided by the production of extracellular enzymes.


5.     Leukocidins :

Produced by some bacteria (e.g. Streptococcus sp. and Staphylococcus sp.)  , that have the ability to destroy phagocytic cells. It causes degradation of lysosomes within leukocytes leading to death of WBC. The hydrolytic enzymes released from the leukocytic lysosomes may damage other cellular structures and thus intensify the infection.


6.     Haemolysins:

Cause the lyses of RBC , e.g. Staphylococcal Betatoxin that destroy the membrane surrounding the RBC.






7.     Coagulases:

They coagulate the fibrinogen in the blood; the resulting fibrin clot may protect the bacterium from phagocytosis and isolate it from other defenses of the host. It is produced by Staphylococcus sp.


8.     Bacterial kinases:

It breakdown fibrin and this dissolve clots formed by the body to isolate the infection e.g. Streptokinase, Staphylokinase


9.     Hyaluronidase :

 It is secreted by certain bacteria and dissolves hyaluronic acid (mucopolysaccharide that holds together certain cells of the body especially in connective tissue). As a result of this action the microorganism will be able to spread more easily from its initial site of infection. It is produced by Streptococcus sp and Staphylococcus sp. and some clostridia that cause gas gangrene.


10.                         Collagenase :

It breaks down the collagen protein which forms the frame work of muscles, produced by several sp. of clostridium facilitating the spread of gas gangrene.



Escape Immune System:


      I.      Resistance to killing by phagocytosis:


  Some pathogens are readily ingested by phagocytes but are resistant to the intracellular killing mechanisms of these cells and may actually multiply intracellularly.

Normally after phagocytosis the phagosome (which contain the bacteria) will fused with the lysosomal granules (which contain enzymes and cationic peptides involved in oxygen–dependent and oxygen–independent bacterial killing mechanism). When fused together they are called phagolysosome.

After ingestion of bacteria by the phagocytic cell there will increase in metabolic reaction involving glycolysis, protein production, membrane phospholipids and in oxygen consumption, this will result in increase in the activity of a no. of enzymes and lead to reduction of molecular oxygen to superoxide O2, hydrogen peroxide, and single oxygen and hydroxyl radical (O and OH) all of them are microbicidal and this is known as oxygen –dependent killing mechanism.

Other enzymes attack independatly to oxygen as lysozyme elastase and hydrolases.


 Different organisms use different strategies for survival phagocytosis.


e.g. Staphylococcus aureus and Neisseria gonorrhoae.








II.  Antigenic variation:

Variation in surface antigen composition during the course of infection provide a mechanism of avoidance of specific immune responses directed at those antigens e.g. Neisseria gonorrhoae which shows variation in an outer membrane protein known as PII. Group A Streptococcus sp. produce up to 75 antigenically different serotype of M protein.


         III.  IgA proteases:

Several sp. of pathogenic bacteria that cause disease on mucosal surfaces produce a protease that specially cleaves IgA The principle antibody type produce at these sites.

E.g. Neisseria meningitides.


Sterilization and Disinfections



It is an absolute term meaning the absence of all microorganisms


It is a process to reduce the no. of contaminating microorganisms particularly those liable to cause infection to a level which is no longer harmful to health.



Disinfectant: are agents used to kill microorganisms capable of producing an infection.

Antiseptic: are agents used to kill microorganisms capable of producing an infection except it is used on living tissue.


Methods of sterilization:

First objects to be sterilized should be cleaned by washing, generally all process used to kill microorganisms has a limited capacity and so the minimum number of organisms the more effective of this process.


            All of this result in reduction of microbial load.

I. Heat:

The time required for sterilization is inversely related to the temperature. Sterilization by heat is the only method of sterilization that is both reliable and widely applicable.

Mechanism of thermal injury:

Usually the lethal effect of heat is attributed to the denaturation and coagulation of protein.

Sterilization by heat is classified in to:

1. Dry heat:

Sterilization by dry heat requires higher temperature and classified into:

a)      Red heat:

It is the main application in the sterilization of inoculating wires and loops and points of forceps (by holding them in the flame of a Bunsen burner until redness).

b)      Flaming:

Direct exposure for few seconds to flame used for scalpels, the necks of flasks. But it is of uncertain efficacy.

c)      Incineration:

An efficient method for the sterilization and disposal of contaminated materials at a high temperature. (Pathological waste materials, surgical dressings. etc).

d)      Hot air sterilizer:

Are used to sterilize materials which can withstand high temperature for long time and are likely to be affected by contact with steam e.g. oils, powders, empty lab-glassware.

The over all cycle of heating up and cooling may take several hours


    Temp.                          Hold-time

                                                               C                                     min

160                                 120

170                                   60

180                                   30

2. Moist heat:

In the presence of steam it is require a lower temperature ( spores are killed at lower temperature by moist heat) used for the sterilization of culture media and other liquids.

  1. Tyndallization:

Used fro sterilization of culture media by exposure to free steam at 100C for a period of 20-45 min on three successive days. The spores that survive the initial heating will germinate in the medium while hold at room temperature during the following day and the resulting vegetative bacteria will be killed when the medium is reheated .

The process will fail to kill bacterial whose spores cannot germinate in the particular medium or under the conditions of storage between heating and it is inapplicable to non-nutrient media.


  1. Boling at 100 C:

Heating in water boiling at 100C for 10 min is sufficient to kill all vegetative bacteria , hepatitis virus and some bacterial spores used for sterilizing metals, glass and rubber items (mainly for surgical equipment).


  1. Steaming at 100C :

Sterilization by exposure to steam (since steam has a temperature of 100C so exposure to this steam for 60 min will kill all microorganisms except bacterial spores and some viruses.


  1. Steaming under pressure (autoclave):

Which provides moist heat at temperature higher than 100C the resulted steam is biocidal. As it condenses on the cooler articles of the load the steam release both thermal energy (latent and sensible heat) and moisture, which together denature microbial proteins.


Factors affecting sterilization by autoclave:

It is the temperature and not the pressure of the steam that confers the sterilizing effect. But the temperature is determined by the pressure. Since increasing in the pressure increase the boiling point temperature, and so high temperature is available


The sterilization hold time is the time required by entire load to be exposed to pure dry saturated steam at the effective temperature in order to ensure sterilization.

Before the start of the sterilization hold time , the load requires to be exposed to steam for an additional period long enough to allow the steam to penetrate in to it and heat it up to effective temperature this is the heat penetration time it is varies with different types of load.


Mechanism of sterilization by pressurized steam:

When the steam comes in contact with the cooler articles in the autoclave it condenses into water on their surfaces.

This condensation has three effects for sterilization

  1. It wet microorganisms so provides the essential condition for killing heat stable organism (bacterial spores).
  2. It liberates the very large latent heat of steam so rapidly heats up the articles.
  3. Cause great contraction in the volume of the steam so promote formation of fresh steam.


Properties of steam:

- Moisture content:

Its high content of moisture provide the conditions for killing of heat stable bacteria (bacterial spores)

- Heat content:

Its latent heat in addition to the sensible heat provides a high temperature necessary for rapid killing of microorganisms.

- Penetration:

The particle size of vaporized water is so small so easily penetrate in to articles.


Test for monitoring of steam sterilizers

In addition to the physical measurements of temperature, pressure and time by thermometers and pressure gauges. Several periodical tests are applied to determine the effectiveness of sterilizers.

The test consists of placing autoclave tap arranged in the shape of a cross (+)  in to the center of the pack and examine the tape after the end of the sterilization cycle , if the sterilization process is good and the steam penetration is good the tap colure is dark, if not the tap colure will not change.

Each brown’s tube contains a red indicator solution that turns green when an adequate temperature has been maintained for sufficient period.

A brown’s tube is placed with the load to be sterilized and immediately inspected after the end of the cycle.

The temperature and pressure reached during each sterilizing cycle of an autoclaving or dry heat oven are recorded on a chart and should be routinely inspected.

Spores strips (include a known number of the spores of Bacillus strearothermophilus they are fixed to different parts of the autoclaves and also placed in the load. after sterilization cycle the strips are cultured in a special broth at 55C for 5 days. Negative culture indicates good sterilization.


II. Irradiation:

  1. Ultraviolet radiation: (240-280 nm optimum 260nm)

Mechanism: formation of pyrimidine diamer so prevent replication and transcription of DNA




  1. Ionizing radiation:


 Formation of free radicals as OH, H and H2O2 that cause microbial cell destruction

Gamma ray radiation:

Source: radiation element   (Cobalt 60)  

Mainly used for sterilizing plastic and other materials that would be damaged by heat (used commercially in sterilizing packaged disposable articles e.g. Gloves and Syringes).


III. Filtration:

Used for the sterilization of heat –labile materials a number of different types of filters have been employed for sterilization. Mainly are membrane filters consisting of porous discs of biological inert cellulose esters.

They are available in pore size of 14 to 0.23 Mm. The 0.22 Mm is he most widely used for sterilization.


IV. Gases:

It is a disinfectant but can be used for sterilization in some cases

Mainly used for decontaminating of rooms, e.g. TB rooms

Formaldehyde is evaporated by heating.


V. Liquids:

In concentration of 2%, exposure for 10 min. It is one of few highly effective cold chemical sterilant used for the sterilization of respiratory therapy equipments, endoscopes, surgical instruments.

It is ten times effective than formaldehyde as bacteriocidal and sporicidal agent and less toxic.



As in sterilization good cleaning is considered as important practice in disinfection process.

I. Heat:

It is the first choice method of disinfection because it is more penetrative, less selective and easy to control.

 Applications of heat disinfectant are mainly depend on temperature and time of exposure.


  1. Pasteurization

Consist of heating the object followed by rapid cooling this will kill most vegetative bacteria, mainly used in dietary industries.

It is of two types

Holder type                          Flash type

63 C for 30 min                    72 C for 2 min


  1. Moist heat

Steam maintained at sub atmospheric pressure at 73C is used to disinfect thermolabile reusable equipment mainly for 20-30 min.


II. Chemical disinfectant:

Factors affecting disinfectant potency:

  1. Concentration of the disinfectant.

Some times low concentration will not affect the microorganism or will not destroy all microorganisms.

  1. The number, type and nature of growth of microorganism.

Increase in number (heavy load) decrease effectiveness of disinfectant.

Type and growth condition (spore forming bacteria are affected by certain type of disinfectant). And in which stage of growth will the disinfectant exert its effect.

  1. Temperature and pH of treatment

The killing of bacteria by chemical agents increases by increase of temperature,

For each 10 degree doubling in death rate.

Each disinfectant is stable and effective ate certain pH so any change will cause alteration in its lethal action.

  1. Presence of extraneous material

Such as organic substances (serum, blood, pus)

They will alter the effect of disinfectant by the adsorption (surface absorption) of the disinfectant leading to the formation of a chemical less active complex.

E.g.: QAC inhibited by soaps and lipids.

  1. Time

Bacteria differ in the time required to be killed.


A. Agent that Damage the Cell Membrane

          They disturb the structural organization of the membrane resulting in interfering with membrane function and altered active transport and energy metabolism.

1. Alcohols

Ethyl alcohol        Isopropyl alcohol


Mode of action:  they disorganize lipid structure by penetrating into the hydrocarbon region also cause denaturation of proteins.

Microbial susceptibility:

G+ve, G-ve, AFB, viruses are susceptible.

Bacterial spores, fungi are resistant.

Factors influencing efficiency:

Adverse effects:


The recommended concentration of alcohol range from 60-80%

2. Phenolic compounds

Mode of action:

Causing leakage of cell contents and irreversible inactivation of membrane-bound oxidase and dehydrogenases.


Phenol (carbolic acid) parent compound not used because of its irritant nature and low activity only used for the testing of newly bacteriocidal agents.

So it is replaced by other agents which are substitutions in the phenol nucleus.

·        Cresols

Mainly used emulsified with soap (commonly known as green soap)

·        Xylenol

·        O- phenylphenol

Microbial susceptibility

G+ve, G-ve, AFB, lipophilic viruses are susceptible.

Bacterial spores, hydrophilic viruses are resistant.

Adverse effect:


3. Surface active disinfectants:

They are substance that causes reduction of the surface-tension.

They are compounds posses both hydrophobic and hydrophilic properties, the hydrophobic portion of the molecule is lipid soluble while hydrophilic portion is water soluble.

They are either Cationic agents or Anionic agents

Quaternary ammonium compounds QAC:


Mode of action:

When bacteria is exposed to this compounds the positive charged group associate with phosphate groups of the membrane phospholipids , while the nonpolar portion penetrates into the hydrophobic interior of the membrane this destruction result in loss of membrane semipermiability and leakage. The agent then may enter and cause denaturation of protein.


                                 CH3                     +

    C16H33                  N             CH3           Br -            Cetrimide


Microbial susceptibility:

G+ve, G-ve, lipophilic viruses are susceptible.

AFB, Bacterial spores, hydrophilic viruses are resistant.

Factors influencing efficiency:

Adverse effects:




Mode of action:

Cause destruction of the lipoprotein framework of the cell membrane.

Microbial susceptibility:

G+ve are susceptible.

G-ve, AFB, Bacterial spores, some viruses are resistant.


B. Agents that denature proteins

They are agents alter the conformation of the protein by denaturation, causing an unfolding of the polypeptide chain so that chains become randomly looped or coiled

There is different chemicals cause denaturation of protein e.g. acids, alkaline, alcohol, acetone and other organic solvent

The organic agents are discussed in membrane altering agents as it is there primary action


Acid and alkaline:


Benzoic acid, lactic acid, citric acid, sodium bicarbonate


Mode of action:

In addition to protein denaturation they change microbial environment



C. Agents that modify functional groups of proteins and nucleic acid

They destroy or alter the functional groups of the enzymes , cell wall , nucleic acid and so prevent there action


1.     Heavy metals:

Soluble salts of mercury, arsenic, silver alter enzyme activity by forming mercaptides with the sulfhydryl group of cysteine.


Mercuric chloride very toxic has limited use.

Mercurochrome less toxic used as antiseptic.

Phenylmercury salts used to control pseudomonas contamination in pharmaceutical and cosmetic preparations.


Highly used as antiseptics


Silver nitrate

Routinely used against gonococci as prophylaxis of Opthalmia neonatorum as will as in the handling of burn patients as topical application of silver nitrate cream


2.     Oxidizing agents:

The most useful agents are the halogens and hydrogen peroxide they inactivate enzymes by converting functional S-H groups to the oxidized S-S form.



They are organic complexes containing iodine trapped within the micelles of an surface active agent.

Microbial susceptibility:

All are susceptible.


Povidone –iodine in conc. 1% soln.

Adverse effects:



Chlorine       Hyochlorite      Chloramine


They all exert their disinfection action by the liberation of free chlorine. When eliminated chlorine is added to water they react with it to form hypochlorous acid which is in acid or neutral solution is a strong oxidizing agent.


                              CL2 +H2O                             HOCL + H+ + CL-


Microbial susceptibility:

All are susceptible.

Factors effecting efficiency:

Adverse effects:




Chlorine dioxide: CLO2

Strong oxidizing agent it is differ from other chlorine compounds is that it does not release hypochlorous acid and its sporecidal is independent to the pH.

Used in water treatment and removal of tastes and odors by breaking down phenolic compounds.


Hydrogen peroxide

It is a slow oxidizing agent used in dilution ranged from 3-6 % as skin disinfectant


D. Dyes


Acridine dyes:

The clinically used types are Proflavine and Acriflavine

Which used in wound antisepsis they interfere with the synthesis of DNA


E. Alkylating agents:

The lethal effects of Formaldehyde, Ethylene oxide and Glutaraldehyde result from their alkylating action on protein, resulting in enzyme modification and inhibition of enzyme activity.


1.  Formaldehyde

In which the carboxyl, hydroxyl or sulfhydryl groups of protein are alkylated by direct replacement of a hydrogen atom with a hydroxymethyl group


Microbial susceptibility:

All are susceptible.



Formaline contain 37-40%




Adverse effect:


2.  Glutaraldehyde


Adverse effects



As for formaldehyde but it is 10 times more effective.

The only available cold sterilant for surgical equipments.




Mode of action:

Cause destruction of cell membrane.


Microbial susceptibility

G+ve are susceptible

G-ve are moderately susceptible

All other microorganisms are resistant



Aqueous and alcohol solution of chlorhexidine 0.02-1 %w/v prepared from stock solution 20% w/v of chlorhexidine gluconate



Mainly as skin disinfectant.



Chemotherapeutic agents & öAntibiotics




Antibiotics: Are chemical substances produced by microorganisms that suppress the growth of other microorganisms and destroy them.


Antimicrobial agents: Term to describe all drugs used to suppress the growth of microorganisms or kill them, including (antibacterial, antiviral, antiparasitic and antifungal).


Antibacterial agents: Include both antibiotics and synthetic chemical compounds that have antibacterial activities.


Bactericidal agents: Drugs used to kill bacteria.


Bacteriostatic agents: Drugs used to suppress the growth and multiplication of bacteria.


Broad spectrum antibiotics: Antibiotics that have antimicrobial activity against large number of types of bacteria.

             (Gram +ve and Gram-ve)


Narrow spectrum: Have antimicrobial activity against only small no of bacteria 

            (Gram +ve or Gram –ve).


Desirable properties of antimicrobial agent:

1.     Selective toxicity

It is an essential property of chemotherapeutic agents, it must inhibit or destroy the pathogen without injury or been toxic to the host.

2.     Bactericidal rather than bacteriostatic

It is for the antimicrobial agent to be bacteriocidal rather than bacteriostatic, so it can be used in different clinical situations (immunodeficiency patients, meningitis…etc)

3.     Resistance

The ideal antimicrobial agents is that one to which the susceptible organisms do not become genetically resistance.

4.     Broad spectrum

It is preferable for the antibiotic to have broad spectrum activity rather than narrow spectrum.

5.     Allergy

Should not be allergenic and with limited side effects.

6.     Solubility

Should be soluble in host tissue.


Antibacterial Agents


Inhibitors of bacterial cell wall synthesis


B-lactam agents:


Mechanism of action:

1. Binding to specific PBPs that serve as drug receptors on bacteria.

2. Inhibiting cell wall synthesis.

3. Activating autolytic enzymes leading to bacterial death.


I. Penicillins

         Source:      Penicillium notatum


Development of penicillins


1.     Natural Penicillins:


Benzyl Penicillin       Penicillin G




Penicillin V


(More stable in gastric acidity and better absorption from GIT)



2.                 Semisynthetic penicillins:

a)          Penicillinase resistant penicillins:



 Methacillin                  Cloxacillin                             

 Nafcillin                      Dicloxacillin    


(Mainly against G +ve bacteria)


b)          Extended spectrum penicillins:


 Ampicillin        Amoxycillin

(In addition to G+ve also are active against G-ve mainly enteric bacteria)

c)           Anti-pseudomonas penicillins:


Carbenicillin              Azlocillin

Ticarcillin                 Mezlocillin 


(Less activity against G-ve, more activity against Pseudomonas sp.)


II. Cephalosporins

            Source: Cephasporium sp.


Development of cephalosporins:


1.     Natural cephalosporin:


Cephalosporin C




      2. Semisynthetic cephalosporins:


First generation:


Cephalexin            Cephalothin

Cephradin             Cephazolin

 (G+ve cocci & many common G-ve bacilli except Enterobacter sp., some proteus sp. and pseudomonas sp)


Second generation:


   Cefaclor                Cefuroxime

  Cefuroxime axetil     Cefamandole

  Cefoxitin               Cefotetan

 (Improved activity against G-ve bacteria but less activity against G+ve cocci. Have more resistance to cephalosporinase enzyme)


Third Generation:


       Cefixim           Cefoperazone       Cefotaxime                

                         Ceftazidime               Ceftriaxone


 (Against G-ve Enterobacteriaceae including multiple resistant isolates & pseudomonas sp.)

Fourth generation:




(Enhanced activity against 3rd generation resistant Enterobacteriaceae) 


Other B-lactam antibiotics:




(No activity against G+ve bacteria but has great effect on multidrug resistant enteric bacteria, Pseudomonas aerugenosa and Nesseria gonorrhoea)




(It is active against all medical important species. (G+ve and G-ve)


B-lactamase inhibitors:


Clavulanic acid        sulbactam


These agents have little or no antibacterial activity . but when combined with a Beta- lactam drug that is susceptible to hydrolysis , they protect the beta-lactam drug from degradation and allow it to exert its lethal effect


It should have the following properties:

1.           Similar structure to the B-lactam drug.

2.      Able to pass through porin channels in G-ve bacteria to destroy B-lactamase enzyme in periplasmic space.


Other cell wall inhibitors:

Vancomycin & Teicoplanine

(Narrow spectrum against G+ve cocci)


(Broad spectrum widely used for UTI)


(Narrow spectrum against G+ve bacteria)


(Broad spectrum used for the treatment of tuberculosis)



Cell membrane inhibitors:




Mechanism of action:

Altering membrane permeability leading to leakage of important metabolites resulting in bacterial death.


Antibacterial activity:

G-ve bacteria mainly Pseudomonas aerugenosa


          Polymyxin B        Colistin



Because of its toxicity, its use limited for localized infections.


Inhibitors of DNA synthesis:


  1. Quinolines


Mechanism of action:

Blocking the DNA replication by inhibit the A subunit of DNA gyrase.



                 Nalidixic acid


Uses: narrow spectrum, Gram –ve bacteria mainly used for treatment of UTI.



           Norfloxacin    Ciprofloxacin


Uses: broad spectrum antibacterial activity (Gram +ve, Gram –ve and Pseudomonas aerugenosa)



2.   Mitronidazole:


Uses: It is antiprotozoal drug but also has antibacterial effects on anaerobic bacteria


Inhibitors of DNA function:




Mechanism of action:

Compete with the P- aminobenzoic acid PABA in the reaction for the synthesis of folic acid.





Mechanism of action:

Inhibit the formation of folic acid by inhibition of dihydrofolate reductase enzyme.



Bacteriostatic, broad spectrum antibacterial drugs. They mainly used in combination in the form of Co-trimoxazole in the treatment of UTIs, RTIs and invasive gastroenteritis.



Inhibition of protein synthesis:


  1. Inhibition of transcription




Mechanism of action:

Bind to RNA polymerase



Mainly used in treatment of tuberculosis, leprosy and sever staphylococcal infections.


  1. Inhibition of translation

a)          Inhibition of the 30S ribosomal subunit


         I. Aminoglycosides

As it is less active in the lack of oxygen it can not be used against bacteriods in addition to naturally resistanct streptococci.

Mechanism of action:

Induce the binding of wrong tRNA leading to the synthesis of false protein.



Neomycin       Kanamycin

Gentamicin     Tobramycin

Amikacin        Spectinomycin



Broad spectrum antibacterial activity



Mainly Ototoxicity and nephrotoxicity


      II. Tetracyclines


Mechanism of action:

Inhibit the binding of tRNA to the 30S subunit


Tertracycline       Oxytetracycline




Broad spectrum, bacteriostatic used against Gram+ve and Gram-ve in addition to mycoplasma, rekettsia and Chlamydia.


Side effect:


     III. Nitrofuranes


Mechanism of action:

Inhibit the synthesis of inducible enzymes and initiation of translation.





Mainly used for the treatment of UTI


b)          Inhibitors of the 50S ribosomal subunits:


I. Chloramphenicol


Mechanism of action:

Inhibit the protein synthesis by inhibition of peptide synthetase.



Broad spectrum, used against G+ve, G-ve, Rickettsia and Chlamydia.



Bone marrow depression


II. Macrolides


Mechanism of action:

Prevent the advancement of the ribosome


  Erythromycin       Clarithromycin                    




bacteriostatic mainly against G+ve cocci for treatment of RTI. in addition has some activity against mycoplasma, chlamydiae and campylobacter sp.

Used as an alternative to penicillin.


   III. Clindamycin & Lincomycin


Mechanism of action:

Similar to erythromycin


Uses: similar to erythromycin but they have more activity against anaerobic organism


   IV. Fucidic acid


Mechanism of action:

Prevent the translocation of tRNA


Use: mainly against G+ve bacteria, treatment of staphylococcal infections.