Staphylococcus

Medium-sized, raised, glistening colonies. The colonies are pigmented, the colour varying

from grey-white, to yellow, or orange. MacConkey pink colonies means the organism

ferments lactose. Gram +, growing on nutrient agar, or Staphylococcus aureus – Gram

positive cocci in clusters or clumps.. clear zone showing production of DNA’ase by Staph.

pseudintermedius

Streptococcus

Gram-positive cocci. Catalase-negative. Attacks sugars by fermentation. Non-motile. Gram

positive cocci in chains. MacConket negative (sensitive to bile salts). Alpha and beta

haemolytic. Step. Uberis positive on Edwards medium ( produce aesculin

Positive catalase test to differentiate Streptococci (-ve) from Staphylococci (+ve). DNA’ase test –

pathogenic Staphylococci break down DNA in the medium because they have the enzyme DNA’ase

(A) Staph epidermidis (-ve), (B) Staph. aureus (+ve), and (C), Staph pseudintermedius (+ve).

Arcanobacterium pyogenes

Gram-positive. Catalase-negative. Non-motile. Attacks sugars by fermentation, but the

reaction is weak. Blood Agar plate which has been incubated with 5% carbondioxide for 24

hours. Small, grey-white, convex colonies.B haemolysis.

Listeria monocytogenes

Gram-positive rods. Catalase-positive. Attacks sugars fermentatively. Motile with

characteristic slow, tumbling and rotary movements. Gram-positive short rodes with rounded

ends. Some cells may be curved. Pinpoint to small, semi-transparent colonies.

Clostridium perfringens

Gram-positive, large, spore-forming rods. Spores are rarely seen as the sporulation process

requires a pH around 7.5. Catalase-negative. Anaerobic, although requirements for

anaerobiosis are not as strict as for many Clostridia. Non-motile. on blood agar grown

anaerobically – note zones of Beta haemolysis

colonial morphology varies considerably, sometimes even within the same culture. The

colonies are small to medium sized and typically grey to grey-yellow and translucent. Some

are smooth and dome-shaped with an entire margin, whilst others are rough with a lobate

margin, still others are flat with an irregular surface and a filamentous margin. Large Gram

positive rods

Clostridium perfringens is non-proteolytic and is not associated with a distinct odour.

sheep blood agar incubated anaerobically, - 2 colonial forms. Colony 1 = Gram negative rods

– Escherichia coli grows anaerobically. Colony 2 = Large Gram positive rods – only grows on

anaerobic plate = Clostridium species (Clostridium perfringens). Gram stain – Gram positive

rods – Clostridium perfringens

Gram negative:

E.coli

Medium-sized grey colonies. The colonies have a characteristic odour. Gram-negative rods.

Catalase-positive and oxidase-negative. Attacks sugars by fermentation and gas is

produced. Motile. Mesophilic. MacConkey pink (resistant) Gram-negative rods with parallel

sides and rounded ends. Escherichia coli growing on nutrient agar. E. coli (C) grows on

MacConkey with pink colonies (lactose fermenter)

sheep blood agar incubated aerobically, Large grey colonies, circular, entire edged 3mm in

diameter – Escherichia coli. Gram negative rods/coccobacilli – Escherichia coli. MacConkey

agar. Large lactose fermenting colonies – Escherichia coli

Salmonella:

Gram-negative rods. Medium-sized, greyish colonies which cause no alteration of the blood.

Catalase-positive and oxidase-negative. Attacks sugars by fermentation and gas is produced

(Salmonella Gallinarum is anaerogenic and Salmonella Pullorum is variable). Motile with the

exception of Salmonella Gallinarum and Salmonella Pullorum. Salmonella typhimurium

growing on nutrient agar. S. typhimurium (D) grows on MacConkey with colourless colonies

(non-lactose fermenter . Salmonella isolation - DCA plate (Desoxycholate Citrate Agar) –

colonies grown from tetrathionate enrichment broth cultures. Black colonies are because of

hydrogen sulphide production. Colonies are colourless because this organism is a non

lactose fermenter on DCA. – Possible Salmonella species

Actinobacillus

Actinobacillus pleuropneumoniae growing on Chocolate (heated blood) agar. A. pleuropneumoniae

(B) does not grow on MacConkey. A. pleuropneumoniae (B) does not grow on Blood agar.

Actinobacillus pleuropneumoniae – growing on blood agar in the presence of Staph. aureus

producing NAD. A. pleuropneumoniae forms tiny colonies using the NAD provided by S. aureus as a

growth factor. Gram-negative rods/coccobacilli

Pasteurella multocida

Gram negative cocci/coccobacilli

Cat bite abscess. Plate 3A – sheep blood agar aerobic, - Pasteurella multocida. No growth MacConkey

Mannheimia haemolytica

blood agar – Beta haemolytic colonies. Gram negative cocco-bacilli

Bordetella bronchiseptica

Grow on MacConkey. Gram negative rods

Pseudomonas aeruginosa

Pseudomonas aeruginosa on blood agar. The ‘holes’ in the initial inoculum is the lytic action of a

bacteriophage killing the bacterial cells. Note the metallic sheen of the colonies.

Pseudomonas aeruginosa sensitivity – this organism is naturally resistant to many antibiotics

sheep blood agar – organism shows metallic glistening colonies with a characteristic smell. The

organism is Pseudomonas aeruginosa. P. aeruginosa is normally resistant to many antibiotics and

causes otitis externa in dogs. Confirm with an oxidase test. MacConkey agar. Pseudomonas

aeruginosa – non lactose fermenting colonies. Gram stain – Gram negative rods – Pseudomonas

aeruginosa.

Proteus mirabilis

Gram-negative rods. Catalase-positive and oxidase-negative. Attacks sugars fermentatively

and gas is produced. Motile. MacConkey agar. This organism is a non lactose fermenter and

spread is inhibited on this medium. aeruginosa on MaConkey agar. This organism is a non

lactose fermenter. Positive oxidase test. Negative control on the left (E. coli). Positive test on

the right (Pseudomonas aeruginosa)

Klebsiella

Medium sized, grey colonies, which cause no alteration of the blood.

Gram-negative short rods. Catalase-positive and oxidase-negative. Attacks sugars

fermentatively and gas is produced. Non-motile. Mesophile. Note the large domed mucoid

colonies. The colonies are large because this organism produces abundant capsular

material. MacConkey agar. This organism is a lactose fermenter. Note the large domed

mucoid colonies. The colonies are large because this organism produces abundant capsular

material

Campylobacter jejuni

Campylobacter jejuni growing on ‘Campylobacter selective agar’ after 48 hours incubation in a

microaerophilic atmosphere. Colonies are moist and spreading. campylobacter jejuni growing on

‘Campylobacter selective agar’ after 48 hours incubation in a microaerophilic atmosphere. Colonies

are moist and spreading. Gram negative spiral bacteria and ‘seagulls wings forms’.

. Plate A – sheep blood agar, plate B, MacConkey agar, plate C, Campylobacter selective agar, plate D,

sheep blood agar incubated anaerobically. Plates have been inoculated with faeces from a pig with

diarrhoea.

Campylobacter selective agar – grown for 48 hours in microaerophilic conditions – the organism is

Campylobacter jejuni. Campylobacter jejuni – Gram negative spiral bacteria and ‘seagulls wings

forms’

Bacteroides fragilis

Growing on blood agar incubated anaerobically. Gram negative rods.

Fusobacterium necrophorum

growing anaerobically on blood agar. The organism shows target haemolysis = double zones of beta

haemolysis. growing anaerobically on blood agar. The organism shows target haemolysis = double

zones of beta haemolysis. pleomorphic Gram negative rods

otitis externa in a dog.

Pustular dermatitis in a dog. Plate 2A – sheep blood agar aerobic, 2B – MacConkey agar aerobic, 2C

DNA’ase plate, 2D – sensitivity test

Yeast:

Microsporum canis

Microsporum canis - Top surface - Sabarauds dextrose agar - White silky surface, yellow reverse

showing through. Reverse surface - yellow reverse. hyphae and microconidia. hyphae and young

macroconidia . hyphae and mature macroconidia. close up hyphae and macroconidia

Candida albicans

Candida albicans plate culture on Sabarauds dextrose agar. Gram positive oval budding yeast (Arrow

= budding yeast)

Malassezia pachydermatis

Plate culture of Malassezia pachydermatis on Sabarauds dextrose agar. bottle shaped elongated

yeasts

Trichophyton mentagraphytes

isolated from the skin of a dog with focal alopecia – soft cottony powdery texture on upper surface –

Sabarauds dextrose agar – Trichophyton mentagraphytes. isolated from the skin of a dog with focal

alopecia – soft cottony powdery texture on lower surface, yellow/orange colour – Sabarauds

dextrose agar – Trichophyton mentagraphytes. Sabarauds dextrose agar – Trichophyton

mentagraphytes. Lactophenol cotton blue prepn – club shaped and round microconidia along the

sides of the hyphae . Trichophyton mentagraphytes Lactophenol cotton blue prepn –spiral hyphae .

Trichophyton mentagraphytes .Lactophenol cotton blue prepn – cigar shaped macroconidia and

round microconidia