Coryneform Bacteria

• Gram positive rods
• Stain irregularly
• Arranged in V forms or palisades
• Non-motile
• Catalase positive
• Non acid-fast
• Non-branching
• Form acid but not gas from carbohydrates

a) Corynebacterium diphtheriae

Is the causative organism of Diphtheria,a localized inflammation of the throat with greyish white adherent excudate and a generalized toxaemia due to the secretion and dissemination of a highly potent toxin

i) Description

• has a characteristic appearance in films from growth on suitable culture media
• the rods are Gram-positive,but are easily decolorized. When stained with Neisser or Albert stains,the volutin granules stain dark purple in contrast to the brown or green counterstain,giving the rods a beaded appearance
• aerobic and facultatively anaerobic,growing best at 37 degrees on a blood or serum containing medium
• Non-motile,non sporing and non-capsulate
• grow best in Loeffler's serum slope which provide morphology for stained films and it grow more slowly on blood or serum tellurite media as greyish to black colonies

ii) Pathogenesis

1. elicit an inflammatory exudate and cause necrosis of the cells of the faucial mucosa
2. infection may spread to the postnasal cavity or the larynx,causing respiratory obstruction
3. the serocellular exudate clots and remains adherent to the fauces and attempts to remove the pseudomembrane leave a raw,bleeding surface
4. the bacilli do not penetrate deeply in the underlying tissues or the blood but they produce a very powerful exotoxin which is spread by the bloodstream and has a special affinity for certain tissues such as heart muscle,nerve endings and the adrenal glands
5. diphtheria toxin is a heat stable polypeptide which is composed of two fragments,denoted A and B. Fragment B is required for transport of fragment A into the cell,where it inhibits polypeptide chain elongation at the ribosome. Inhibition of protein synthesis is probably responsible for both the necrotic and neurotoxic effects

iii) Clinical aspects of infection

• the incubation period is 2-6 days
• patients present with malaise,fatigue,fever and sore throat,though this is not major feature
• anterior nasal diphtheria presents with a thick nasal discharge and intoxication is rare
• tonsillar ingection presents with a thick,adherent green pseudomembrane on one or both nostrils
• in pharyngeal infection,the pseudomembrane may involve the soft palate,uvula,pharyngeal wall and nasal mucosa
• oedema,heat amd tenderness may occur in the anterior tissues of the neck and there is local lymphadenopathy
• electrocardiogram abnormalities are common in the first week of the illness. Visual disturbance,difficulty in swallowing and paralysis of the arms and legs also occur but usually resolve spontaneously

iv) Treatment

• therapy involves isolation of patients,bed rest,appropriate antibiotics and early administration of antitoxin
• supportive therapy may be required such as intubation and ventilation for airway obstruction
• C.diphtheriae is sensitive to penicillin,tetracycline,rifampicin and clindamycin
• treatment should not await laboratory confirmation if there is a strong clinical suspicion as the fatality rate is directly related to the period of delay before giving antitoxin

Other medically important Coryneforms

b)Corynebacterium ulcerans

• causes bovine mastitis and sporadic infection mainly due to infected animals or milk
• almost all infections are pharyngeal,though occasional soft tissue infections occur
• infection usually takes the form of acute pharyngitis,pseudomembranes may form and cardiac or neurological complications can occur
• antitoxin is one of the treatment but its efficacy is not proven
• erythromycin is the antibiotic of choice but several other agents can also be used

c) Arcanobacterium haemolyticum

• the main clinical infection are acute pharyngitis which is usually sporadic and chronic skin ulcers
• most patients are yound adults who present sore throat,some have cervical lymphadenopathy and may develop a scarlatiniform rash
• erythromycin is effective but mild pharyngitis settles in a few days without therapy
• Arcanobacterium haemolyticum is sensitive to penicillin too but clinical failures have occurred

d) Corynebacterium pseudotuberculosis

• causes a causeous lymphadenitis in sheep
• infection presents as a subacute or chronic lymphadenitis involving the axillary or cervical nodes,often with systemic symptoms
• erythromycin,tetracycline or penicillin have been used,however,surgical drainage or excision may be necessary

e) Corynebacterium minutissimum

• associated with the skin condition erythrasma,a localized infection of the stratum corneum which produces red scaly patches in the toewebs,groins and axillae

f) Corynebacterium jeikeium

• found on mucosal surfaces and the skin of a few healthy individuals
• neutropenic patients and those receiving antibiotics,are commonly colonized. It has been isolated from blood,tissue and cerebrospinal fluid (CSF) in patients with neutropenia and after cardiac surgery
• large infected groups are patients with haematological malignancy receiving cytotoxic chemotherapy
• usually resistant to penicillin,cephalosporins and aminoglycosides but sensitive to glycopeptides
• vancomycin and teicoplanin are effective in treating line-associated infection in neutropenic patients
• combinations of penicillin or vancomycin with gentamicin are synergic in vitro and have been used to treat infective endocarditis

Listeria

• non sporing
• Gram positive bacilli
• Main pathogenic species is Listeria monocytogenes,which chiefly affects pregnant women,their babies and immunosuppressed adults,esp the elderly

a) Description

• short rods
• grow well on a wide variety of laboratory media
• some species are belta-haemolytic and others are non-haemolytic
• CAMP test,bacteriophage typing,plasmid profiling,multilocus enzyme electrophoresis and random amplified polymorphic DNA analysis are used in the classification of different species

b) Pathogenesis

• all strains that are pathogenic to mice produce a haemolysin,listeriolysin O,which is antigenically similar to Streptolysin O and it is important for bacterial survival after phagocytosis and its production is regulated by extracellular iron concentrations
• several other toxins are produced : a factor which promotes bacterial entry into the cells,a cytolysin and enzymes which protect the bacteria from the harmful effects of reactive oxygen radicals such as superoxide dismutase and catalase
• oral administration of bacteria to germ-free animals causes gut wall infection and occasional spread the liver and spleen,but colonization of the bowel is inhibited by the normal gastro-intestinal flora. In humans,gastro-intestinal disease,causing low gastric pH,disrupted bowel flora or damaged bowel tissues,may help to establish listeria in the bowel
• lysozyme can lyse soem strains of listeria and human neutrophils and non-activated macrophages can phagocytose and kill the bacteria. T lymphocytes confer protection by attracting monocytes to infectious foci

c) Clinical aspects of infection

i) Infection in pregnancy and the neonate

• maternal listeriosis before 20 weeks of pregnancy is rare. The mother is usually previously well and having a normal pregnancy. Pregnant women often have very mild symptoms and may be asymptomatic until the delivery of an infected infant. Chills,fever,back pain,sore throat and headache are cardinal features of infection,but conjunctivitis,drowsiness and other symptoms are sometimes present
• symptomatic women may have psoitive blood cultures. High vaginal swabs (HVSs),stool cultures and mid-stream urine samples are generally of little help in antepartum diagnosis
• with the onset of fever,fetal movements are reduced and premature labour occurs within about 1 week. There may be a transient fever during labour and the amniotic fluid is often discoloured or meconium stained
• culture of the amniotic fluid,placenta or HVS post delivery usually yields L.monocytogenes
• fever resolves soon after birth and the HVS is usually culture-negative after about 1 month
• usually,the outcome of infection for women is benign while the outcome for the infant is more variable
• maternal infection,sometimes does not infect the offspring and even progress to placental infection without ill effects for the fetus
• early neonatal listeriosis is predominantly a septicaemic illness,contracted in utero. In contrast,late neonatal infection is predominantly meningitic and may be associated with hospital cross-infection

ii) Adult and juvenile infection

• listeriosis in children older than 1 month is very rare except with the children with underlying disease
• in adults,the main syndromes are CNS infection,septicaemia and endocarditis

iii) Bacteraemia and endocarditis

• is more common in men than in women and occurs most often in patients with haematological malignancy or renal transplants
• infective endocarditis is twice as common in men as in women. The main predisposing factor are prosthetic valves or damaged natural valves but some patients belong to other risk groups

d) Treatment

• susceptible to a wide range of antibiotics in vitro including ampicillin,penicillin,vancomycin,tetracyclines,chloramphenicol,aminoglycosides and co-trimoxazole
• ampicillin and penicillin are probably equivalent agents for the treatment of meningitis. Cotrimoxazole is an excellent alternative
• chloramphenicol was formerly used in listeria meningitis but when used alone,it is less effective than penicillin or ampicillin and results in relapses

Erysipelothrix

• aerobic
• non-sporing
• Gram-positive bacilli
• short - 1 to 2 micrometre
• grow on blood agar or in glucose broth is improved by incubation in 5-10% carbon dioxide
• species E.rhusiopathiae causes economically important disease in domestic animals,notably pigs
• human infections are rare,but include a form of cellulitis (erysipeloid),which occasionally becomes diffuse,septicaemia and endocarditis
• penicillin and other belta-lactam antibiotics are effective
• erythromycin and clindamycin offer suitable alternatives but E.rhusiopathiae is resistant to vancomycin


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