If you receive a swab from a wound, pus, bite, or from the eye or ear, and the bacteria in question is known to survive in aerobic conditions, which medias would you perform the preliminary cultures on?
- HBA
- CBA
- MAC
Give the name of a selective media that you would use to isolate faecal samples
MAC (MacConkey agar)
Describe MacConkey agar
- Is considered both selective and differential
- Selectively isolates gram negative and enteric bacilli (the bile salts are selective for enteric organisms)
- Differentiates these enteric organisms based on lactose fermentation ->
Lactose fermenter = PINK
Non-lactose fermenter = CLEAR
Describe the features of S. aureus
Morphology:
- Gram positive
- Cocci
- Clusters
Media & details on media:
- HBA - forms yellow colonies, beta haemolytic
- CBA - forms yellow colonies
- MAC - no colonies formed (as the bile salts are selective for gram negative enteric organisms)
- MSA - forms yellow colonies
Test results:
- Catalase +
- Bactistaph +
- All Staphylococci are facultative anaerobes
Key virulence factors:
- Produces toxins (haemolysins, leukotoxins, leukocidin)
- Catalase (enhances survival in phagocytes)
- Invasins (hyaluronidase)
Effects of infection:
- Toxic shock syndrome
- Septic shock
Treatment:
- Antibiotics - Vancomycin and Tetracycline
Describe the features of Streptococcus pneumoniae.
Morphology:
- Gram positive
- Cocci
- Paired
Media & details on media:
- HBA - forms colonies, alpha haemolytic
- CBA - forms colonies
- MAC - no colonies formed (as the bile salts are selective for gram negative enteric organisms)
- Facultative anaerobe
Key virulence factors:
- CAPSULE
- Invasins
- Adhesins
- Exotoxins
Effects of infection:
- Pneumonia
- Meningitis
- Bronchitis
People most susceptible to infection:
- Elderly people
- Children under the age of 5
- Immunocompromised individuals
Treatment:
- Pneumococcal vaccines
- Antibiotics - Penicillin
How do you confirm a S. pneumoniae infection?
- Obtain a swab from a patient and perform a gram stain (should observe Gram positive cocci in a paired arrangement)
- Culture on a HBA plate (should observe alpha haemolysis)
- Place a optochin disc on the HBA (should be sensitive to optichon)
- Capsule stain (Maneval’s stain) to confirm that it is S. pneumoniae
Define what a fastidious organism is
- An organism that has complex nutritional requirements
Describe what CBA is
- Chocolate blood agar
- Is non-selective
- A variant of the blood agar plate, containing red blood cells which have been lysed slowly
- Used for growing fastidious respiratory bacteria, such as Haemophilus influenzae
Describe the oxidase test
- Used to identify bacteria that produce cytochrome c oxidase (enzyme of the bacterial electron transport chain)
- Positive results indicate that the bacteria is aerobic
+ve test = dark purple colour
-ve test = no colour change
How do you differentiate between Staphylococcus and Streptococcus?
- Catalase test
- Staphylococci are catalase positive, while Streptococci are catalase negative
Describe the features of Streptococcus pyogenes
Morphology:
- Gram positive
- Cocci
- Short chains
Media & details on media:
- HBA - forms colonies, beta haemolytic
- CBA - forms colonies
- MAC - no colonies formed (as the bile salts are selective for gram negative enteric organisms)
Test results:
- Catalase -
- Facultative anaerobe
Key virulence factors:
- M protein
- Hyaluronidase
- Exotoxins (Streptolycin O, Streptolycin S)
- Invasins (Streptokinase)
Effects of infection:
- Toxic shock syndrome
- Scarlet fever
Treatment:
- Antibiotics - Penicillin
How do you confirm a S. pyogenes infection?
- Gram stain (should observe gram positive cocci in short chains)
- Perform a catalase test (should be catalase negative)
- Observe colonies on HBA plate (should be beta haemolytic)
- Place bacitracin disc on HBA (should be bacitracin sensitive)
Describe the features of Salmonella enterica
Morphology:
- Gram negative
- Rods
- Flagellated (motile)
Media & details on media:
- MAC - clear colonies formed (ie lac negative)
- XLD - red colonies with black centres
Test results:
- Oxidase -
- Facultative anaerobe
Key virulence factors:
- Type III secretion system
- Flagella
- Invasins
- Enterotoxins
Effects of infection:
- Typhoid
- Gastroenteritis
Treatment:
- Antibiotics
Prevention:
- Hand washing
- Cooking food well
How do you confirm an infection by Salmonella enterica?
- Grow on MAC agar (should be Lac negative - Clear)
- Perform an oxidase test (should be oxidase negative)
- Grow on XLD agar (should be red colonies with BLACK centres)
- Identify using agglutination (should agglutinate)
Describe Vibrio cholerae
Morphology:
- Gram negative
- Rods
- Flagellated (motile)
Media & details on media:
- MAC - clear colonies formed (ie lac negative)
- TCBS - yellow colonies
Test results:
- Oxidase +
- Facultative anaerobe
Key virulence factors:
- Cholera toxin
- Flagella
- Pilus
Effects of infection:
- Cholera
Treatment:
- Antibiotics
Prevention:
- Water purification
- Proper sanitation
How do you confirm an infection by Vibrio cholera?
- Grow on MAC agar (should be Lac negative - Clear)
- Perform an oxidase test (should be oxidase positive)
- Grow on TCBS agar (should observe yellow colonies)
Describe XLD agar
- Xylose lysine deoxycholate agar
- Used to isolate Salmonella and Shigella species
Red colonies with black centres - Salmonella (forms black spot due to the production of H2S)
Red colonies - Shigella
Yellow colonies - normal gut flora
Describe TCBS agar
- Thiosulfate citrate bile sucrose agar
- Selective media for pathogenic Vibrio sp.
Yellow - Vibrio cholera
Green - Other Vibrios
Describe Mycobacterium tuberculosis
Morphology:
- Can appear gram poitive or negative
- Small rod
Media & details on media:
- Lowenstein jensen media - brown colonies
Tests:
- Acid fast +
- Catalase +
- Strict aerobe
Key virulence factors:
- Mycolic acid
- Slow generation time
- Intracellular growth
Effects of infection:
- Tuberculosis
Treatment:
- Drugs
How do you confirm an infection by M. tuberculosis?
- Perform an acid-fast stain (should be red)
- Observe on Lowenstein-jensen medium (3-4 weeks incubation due to slow doubling time of TB; should observe brown colonies)
What are acid fast stains?
- Ziehl Neelsen or Kinyoun’s stain
Differential stains used to detect acid- and alcohol- fast organisms - Useful for detection of Mycobacterium sp.
Red - acid fast organisms
Blue/green - non acid fast organisms
