Microbiology Lab

Staphylococcus aureusGram-positive

Violet cocci in grape-like clusters

Catalase positive · coagulase positive · oxidase negative

Carried on skin and in the nose. Grows in food that is handled and left warm, and its toxin survives cooking.

Staphylococcus epidermidisGram-positive

Violet cocci in clusters, like S. aureus

Catalase positive · coagulase negative

Normal skin flora, and the commonest contaminant from hands. Coagulase is what tells it from S. aureus.

Streptococcus pyogenesGram-positive

Violet cocci in chains

Catalase negative · clear (beta) haemolysis on blood agar

Throat and skin infections. Catalase separates streptococci from staphylococci.

Enterococcus faecalisGram-positive

Violet cocci in pairs and short chains

Catalase negative · grows in bile and 6.5% salt

Lives in the gut, so in water it is a sign of faecal contamination.

Bacillus cereusGram-positive

Large violet rods, often in chains; forms spores

Catalase positive · spores survive cooking

Its spores survive in cooked rice and pasta, then grow if the food is cooled slowly or kept warm.

Listeria monocytogenesGram-positive

Short violet rods, singly or in short chains

Catalase positive · tumbling motility · grows in the fridge

Grows at refrigerator temperatures in ready-to-eat foods. Dangerous in pregnancy and for the frail.

Clostridium perfringensGram-positive

Large, box-shaped violet rods

Catalase negative · grows only without oxygen · forms spores

Big batches of meat and gravy cooled too slowly. Needs anaerobic incubation to grow.

Escherichia coliGram-negative

Pink rods, singly

Oxidase negative · ferments lactose · indole positive

Lives in the gut: in water or food it marks faecal contamination. Some strains, such as O157, cause severe illness.

Salmonella entericaGram-negative

Pink rods, singly

Oxidase negative · does not ferment lactose · makes hydrogen sulfide (black centres on XLD)

Poultry, eggs and raw produce. Must be absent in 25 g of a ready-to-eat food.

Pseudomonas aeruginosaGram-negative

Slender pink rods

Oxidase positive · green pigment · grape-like smell

Thrives in water systems, sinks and cosmetics. Oxidase separates it from the gut bacteria.

Campylobacter jejuniGram-negative

Small pink curved or S-shaped rods

Oxidase positive · grows at 42 °C in low oxygen

The commonest cause of bacterial food poisoning; raw poultry is the usual source.

Legionella pneumophilaGram-negative

Thin rods that stain faintly pink

Won't grow on ordinary agar: needs buffered charcoal yeast extract agar

Grows in warm water systems and cooling towers, and infects through breathed-in spray.

Yeasts (Candida, Saccharomyces)Yeast

Large oval cells, many with a bud, staining violet

Much larger than bacteria · grows on acidic agar at 25 °C

Spoils juices, sauces and yoghurt: gas, off-flavours and blown packs.