Gram-positive bacilli Medical Assignment Help

CORYNEBACTERIA INFECTION

Corynebacterium diphtheriae is a Gram-positive, clubshaped bacillus. Three morphological varieties are recognized – mitis, intermedius and gravis. Of these, mitis is generally associated with mild infections. Only corynebacteria exposed to the bacteriophage f3, which carries the tox” gene are capable of toxin production. The toxin has two subunits, A and B. Subunit A is responsible for clinical toxicity. Subunit B serves only to transport the toxin component to specific receptors, present chiefly on the myocardium and in the peripheral nervous system. Humans are the only natural hosts.

Diphtheria

Diphtheria caused by C. diphtheriae occurs worldwide. Its incidence in the West has fallen dramatically following widespread active immunization. Transmission is mainlythrough airborne droplet infection and rarely through fomites.

CLINICAL FEATURES

The incubation period varies from 2 to 7 days. Diphtheria is essentially a disease of childhood. The manifestations may be regarded as local (due to the membrane) or systemic(due to exotoxin). The presence of a membrane, however, is not essential to the diagnosis. The illness is insidious in onset and is associated with tachycardia and low-grade fever. If complicated by infection with other bacteria such as Strep. pyogenes, fever is high and spiking. NASAL DIPHTHERIA is characterized by the presence of a unilateral, serosanguinous nasal discharge that crusts around the external nares.

HARYNGEAL DIPHTHERIA is associated with the greatest toxicity and is characterized by marked tonsillar and pharyngeal inflammation and the presence of a membrane. This tough greyish-white membrane is formed byfibrin, bacteria, epithelial cells, mononuclear cells and polymorphs, and is firmly adherent to the underlying tissue. Regional lymphadenopathy is prominent and produces the so-called ‘bull-neck’.

LARYNGEAL DIPHTHERIA is usually a result of extension of the membrane from the pharynx. A husky voice, a brassy cough, and later dyspnoea and cyanosis due to respiratory obstruction are common features.
Clinically evident myocarditis occurs, often weeks later, in patients with pharyngeal or laryngeal diphtheria. Acute circulatory failure due to myocarditis may occur in convalescent individuals around the tenth day of illness and is usually fatal. Neurological manifestations may occur either early in the disease (palatal and pharyngeal wall paralysis) or several weeks after its onset (cranial nerve palsies, paraesthesiae, peripheral neuropathy or, rarely, encephalitis).

CUTANEOUS DIPHTHERIA is increasingly being seen in association with burns and in individuals with poor personal hygiene. Typically the ulcer is punched-out with undermined edges and is covered with a greyish-white to brownish adherent membrane. Constitutional symptoms are uncommon.

DIAGNOSIS

This must be made on clinical grounds since therapy is usually urgent and bacteriological results of culture studies and toxin production cannot be awaited.

REATMENT

The patient should be isolated and bed rest advised. Antitoxin therapy is the only specific treatment. It must be instituted rapidly to prevent further fixation of toxin to tissue receptors, since fixed toxin is not neutralized by antitoxin. Depending on the severity, 20000-120000 units of horse-serum antitoxin should be administered intravenously after an initial test dose to exclude any allergic reaction. There is a risk of anaphylaxis immediately after antitoxin administration and of serum sickness 2-3 weeks later. Antibiotics should be administered concurrently to eliminate the organisms and thereby remove the source of toxin production. Penicillin is given for 1 week.

PREVENTION

Diphtheria can be effectively prevented by active immunization in childhoodAll contacts of the patient should have throat swabs sent for culture; those with a positive result should be treated with penicillin or erythromycin and active immunization or a booster dose of toxoid given.

lISTERIAL INFECTION

Listeria monocytogenes is a non-spore-forming, facultatively anaerobic bacillus that is motile at 20-25°C. It grows optimally at 30-37°C but can multiply at 4°C and survive heating to 60°C. It is found worldwide and is widely disseminated in the environment. Listeriosis predominantly occurs perinatally but may occasionally occur in adults, particularly the immunocompromised and elderly. It causes abortions, septicaemia and meningitis. The mortality rate is high. Concern has arisen because of the increasing number of food-borne outbreaks in the past 10 years. Foods most commonly implicated are raw vegetables, coleslaw, milk, non-pasteurized soft cheeses, chicken and pate. Cook-chill catering has come under scrutiny, the implication being that the organism can survive if reheating is inadequate. The organism can multiply in the refrigerator if temperatures are not kept below 4°C Diagnosis is established by blood or CSF culture. Treatment is with ampicillin and gentamicin. Erythromycin, co-trimoxazole or rifampicin are alternatives.

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