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