12 Diagnosis microbiology

Question 1

Nutrient agar *

Answer 1

• Pigmentation easily observed:
  
  • Golden-yellow colonies of Staph aureus *
  • Diffusible blue-green pigments of Pseudomonas *
  • Red colonies of Serratia marcescens

Question 2

Blood agar

Answer 2

• Nutrient agar + 5-10% sheep blood
• Hemolytic properties can be distinguished
• **Initial culture of specimens **
• Streptococci categorization *

Question 3

Chocolate agar *

Answer 3

• Heated blood agar
• 2 imp bacteria that can grow ONLY on this:
  
  • Haemophilus influenzae
  • Neisseria gonorrhoeae

Question 4

MacConkey agar (MA) *

(Differential media)

Answer 4

Differentiates bacteria based on utilization of Lactose

note:

• Stool cultures are routinely plated on MacConkey agar:
  
  • Normal flora like E.coli produce pink lactose fermenting colonies
  • Pathogens like Salmonella, Shigella produce colourless colonies and can easily be picked out for isolation and testing
• MA also routinely used for plating urine cultures:
  
  • E.coli – pink colonies
  • Proteus (also a Urinary tract pathogen) is colourlesson MA

MA supports growth og only enteric gram negative bacteria
Gram positive bacteria do not grow on MacConkey agar

Question 5

Salmonella-Shigella agar (SS agar)

(Selective media *)

Answer 5

• To isolate these 2 pathogens from stool cultures
• Contains bile salts that select growth of these 2 pathogens while supressing normal flora

note:

• Bile Salts, Sodium Citrate and Brilliant Green inhibit Gram-positive bacteria, most coliform bacteria, and inhibit swarming Proteus spp., while allowing Salmonella spp. And Shigella spp to grow.
• Sodium Thiosulfate and Ferric Citrate permit detection of hydrogen sulfide by the production of colonies with black centers.

Question 6

Single enzyme tests *

Answer 6

• Detection of various enzymes for the identification of bacteria
• Detection of enzymes aids in presumptive identification
  
  • Catalase test
    
    • Staphylococci = catalase +
    • Streptococci = Catalase –
  • Oxidase test
    
    • Oxidase + (Pseudomonas, Helicobacter)
    • Oxidase – (all Enterobacteriaceae (E.coli))
  • Urease test
    
    • Proteus +
    • Helicobacter +
  • Coagulase test
    
    • Staph aureus = coagulase +
    • Staph epidermidis = coagulase –

Question 7

Catalase test *

Answer 7

• Enzyme catalyses the conversion of hydrogen peroxide to water with release of O2 gas
• Key test for differentiating Gram positive cocci
• Staphylococci = catalase +
• Streptococci = Catalase –

Question 8

Oxidase test *

Answer 8

• Detects cytochrome oxidase produced by aerobes
• Used in differentiation of gram negative bacteria
• Oxidase +
  
  • Pseudomonas, Helicobacter
• Oxidase –
  
  • all Enterobacteriaceae (E.coli)

Question 9

Urease test *

Answer 9

• Urease hydrolyses urea to CO2 and ammonia
  
  • alkalinizes medium
  • indicator changes
  • medium changes colour
• Urease +
  
  • Proteus
  • Helicobacter

Question 10

Coagulase test *

Answer 10

• causes clot formation when bacterial isolate is incubated with plasma
• Staph aureus = coagulase +
• Staph epidermidis = coagulase –