Tuesday, February 21, 2017

Micro-organism series: Staphylococcus aureus

Hello awesomites here is our first bacteria of micro-organism series - Staphylococcus aureus

Aerobes and facultative anaerobes
Diameter: 1 micrometer
Gram profile: Gram positive Cocci
Arrangement: Grape like clusters
Motility: Non-motile
Sporeing or not: Non-sporing
Capsule: Usually capsulated. Non-capsulated strains have small amount of capsular material on the surface

Cultural characteristics:

Ordinary media: They grow readily with temperature range of 10-42°C ( Best at 37°C) and pH around 7.4 - 7.6

Nutrient agar:
You can remember how colony looks by the name STAPHYLOCOCCUS it self.
S - Smooth
O - Opaque
C - Circular
C - Convex
Easily emulsifiable.
It produces golden yellow pigment. Pigment production occurs best at 22℃ and only in aerobic culture. Growth represents "Oil-paint appearance".

Blood agar: Shows beta type of hemolysis when incubated under 20-25% carbon dioxide.

MacConkey agar: Pink colour small colonies (Because it ferments lactose!)

Selective media:
Ludlam's medium - Lithium chloride and tellurite.
Mannitol salt agar or salt milk agar- 1%Mannitol, 7.5%NaCl, Phenol red indicator.
Columbia colistin nalidixic acid agar.

Liquid media: Turbidity.

Biochemical Reactions:
Catalase : Positive
Oxidase : Negative
Mannitol: Positive
Indole: Negative
MR andVP: Positive
Phosphatase : Positive (Useful for screening as S epidermidis is negative or slightly positive )
Urease : Positive

Produces thermonuclease enzymes.

Virulence factors :
Extracellular enzymes :
Coagulase (Most virulent)

5 types of Cytolytic toxins: Alpha, Beta, Gamma, Delta and Leucocidins
Alpha: Most important hemolysin.
Beta: It is sphingomyelinase. Exhibits hot and cold phenomenon.
Leucocidin and gamma lysin is grouped under synergohymenotropic toxins.

9 types of enterotoxins: Responsible for staphylococcal food poisoning. Causes nausea, vomiting and diarrhoea within 2-6 hours.
A, B, C1, C2, C3, D, E, H and I.

Super antigens:
Toxic shock syndrome toxin (TSST)
(What are superantigens and how do they work?)

Disease caused:
Remember " SOFT PENIS"
S - Skin infections
O - Osteomyelitis and arthritis
F - Food poisoning
T - Toxic shock syndrome
P - Pharyngitis and Pneumonia (Rarely)
E - Endocarditis
N - Necrotizing fascitis
I - Impetigo
S - Sepsis

Laboratory diagnosis:
Specimen and lesions
Pus - Suppurative lesions
Sputum - Respiratory Infections
Blood - Septicaemia
Urine - UTI
CSF- Meningitis
Faeces - Food poisoning
Collection and transport :
Sterile containers should be used
Sterile swabs for specimen from nose/perineum.
Direct microscopy :
Gram stained smears is useful in case of pus, cocci in clusters may be seen.

Sensitive to penicillin: Penicillin G is DOC (Drug of choice)
Penicillin allergy: Cefazolin is DOC
Methicillin resistant S.aureus: Vancomycin
Vancomycin resistant: S.aureus: Streptogramins, Linezolid, Telavancin.

That's all!
Finally done!
Stay awesome :)
~Khushboo shaikh,IkaN,and ojas


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