Actinomycetes and Non Sporing Anaerobes Questions and Answers

Answer: B

Correct answer: B melaninogenicus

The pigmented Bacteroides, of which B. melaninogenicus is the type, accumulate protoporphyrin from haemin in the medium, and long before the black pigment is visible in the colony the protoporphyrin fluoresces a brick-red under Wood’s ultraviolet light. The finding allows a presumptive identification within 48 hours, well before the colonies blacken, which may take a week. The organism is now classified in the genus Prevotella.

Answer: B

Correct answer: Metronidazole

Metronidazole is reduced to a cytotoxic intermediate only in an anaerobic environment, so it is active against virtually all obligate anaerobes and is inactive against aerobes. Bacteroides fragilis, the dominant pathogen below the diaphragm, is reliably sensitive to it and commonly resistant to penicillin through its beta-lactamase. Gentamicin and aztreonam are inactive against anaerobes altogether, aminoglycosides requiring oxygen-dependent transport into the cell, and nalidixic acid is a urinary agent against Gram-negative aerobes.

Answer: A

Correct answer: Gram+ve organism

Actinomyces israelii is a Gram-positive branching filamentous bacterium. Its name and its mycelial growth led to its being taken for a fungus for many years, but its cell wall structure, its width of about 1 micrometre, and its sensitivity to penicillin all identify it as a bacterium. It is not acid-fast, which is the practical test separating it from Nocardia, and it is certainly not Gram-negative.

Answer: C

Correct answer: Gram-positive branching filaments with clubbed ends at the periphery

A sulphur granule is a colony of the organism growing within the pus. Crushed between two slides and Gram-stained, its centre shows a tangled mass of Gram-positive branching filaments about 1 micrometre wide, and at the edge the filaments end in radiating, club-shaped swellings of host-derived material - the classic sunray appearance. The granules are yellowish and gritty, and finding them in pus draining from a chronic sinus is effectively diagnostic of actinomycosis.

Answer: D

Correct answer: Mycetoma

Mycetoma is defined by the triad of a chronic painless swelling, multiple discharging sinuses, and granules in the discharge, usually on the foot after an implanting injury. Sporotrichosis spreads along the lymphatics as a chain of nodules that may ulcerate but does not form granule-bearing sinuses; cryptococcosis presents as meningitis; and histoplasmosis is a systemic disease of the reticuloendothelial system. Sinuses discharging granules should send the specimen straight for granule microscopy.

Answer: B

Correct answer: microaerophilic conditions in presence of 5% CO₂ and PH6

Lactobacilli are aerotolerant rather than strictly anaerobic. They grow best in a microaerophilic atmosphere enriched with about 5 per cent carbon dioxide, and they prefer an acid medium of around pH 6 - a good deal lower than the pH 7.2 to 7.4 at which most pathogens are cultured. Rogosa and tomato juice agars are deliberately acidified for this reason, and the same acid tolerance is what allows the organism to survive in the vagina in the acid conditions it creates for itself.

Answer: A

Correct answer: An intrauterine contraceptive device left in place for years

Pelvic actinomycosis is strongly associated with a long-retained intrauterine device, the plastic providing a surface on which the organism can grow in a low-oxygen microenvironment. It may present as a chronic pelvic mass with adherent bowel that mimics malignancy, and Actinomyces-like organisms are sometimes reported incidentally on a cervical smear from an asymptomatic user. Removal of the device, with prolonged penicillin if there is established disease, is the treatment.

Answer: B

Correct answer: Mobiluncus

Mobiluncus species are curved, motile, gram-variable anaerobic rods found in bacterial vaginosis along with Gardnerella vaginalis and various anaerobes, replacing the normal lactobacillary flora. Their comma or gull-wing shape on a wet mount or Gram film is a useful pointer. Veillonella is a Gram-negative coccus, and Bifidobacterium and Eubacterium are Gram-positive rods of the gut and are not implicated in this syndrome.

Answer: B

Correct answer: Capsular polysaccharide, which promotes abscess formation

The capsular polysaccharide complex of B. fragilis is the reason a comparatively minor member of the faecal flora becomes the commonest anaerobe in clinical specimens. It resists phagocytosis, and it is by itself sufficient to induce an abscess in an experimental animal, so the organism converts a spilled bowel content into a walled-off collection. The capsule also explains why surgical drainage is as important as antibiotics. Bacteroides is non-motile and non-sporing, and it produces no exotoxin A.

Answer: B

Correct answer: Bacillus cereus

The short incubation of one to six hours with vomiting as the dominant symptom, following a meal of fried rice, is the emetic syndrome of Bacillus cereus. Spores surviving boiling germinate in rice left standing at room temperature, and the preformed, heat-stable cereulide toxin is not destroyed by the brief reheating of frying. Staphylococcus aureus food poisoning is the closest differential, also short and vomiting-predominant, but the rice history points to B. cereus. Clostridium difficile follows antibiotics and causes diarrhoea rather than vomiting.