Beta hemolytic colony of Staphylococcus aureus on Blood agar and this

Beta hemolytic colony of Staphylococcus aureus on Blood agar and this organism related informations

Beta hemolytic colony of Staphylococcus aureus on Blood Agar

Beta hemolytic colony of Staphylococcus aureus on blood agar as shown above image and it has following features-On 5% defibrinated sheep blood agar, colonies are circular, convex, smooth, shining, opaque, emulsified easily. On blood agar plates, colonies of Staphylococcus aureus are frequently surrounded by a clear zone of hemolysis i.e. beta-hemolysis. The golden appearance of colonies of some strains is the etymological root of the bacteria’s name; aureus meaning “golden” in Latin and observed clearly on nutrient agar.
S. aureus may occur as a commensal on human skin. It also occurs in the nose frequently ( about a third of the population) and throat less commonly. The occurrence of this organism under these circumstances does not always indicate infection and therefore does not always require treatment.

Morphology

Spherical (0.8µm)
Gram-positive cocci in the cluster
Non-motile
Non-capsulated
Non-sporing

Culture

Uniform turbidity in peptone water
Pigmented colonies ( golden yellow), pinhead size, circular, convex, smooth, shining, opaque, emulsified easily on nutrient agar.
5 % sheep blood agar:-In blood agar beta-hemolytic colony

Biochemical Test

It ferments with acid production only in glucose, lactose, sucrose, and mannitol.
Catalase test: Positive
Coagulase test: Positive ( both slide and tube)
Sodium chloride tolerance (7.5%)
Liquefies gelatin
Egg yolk medium lipolytic activity: Opalescence around the colony
DNase test: Positive

Resistance

The thermal death point of Staphylococcus aureus is 60°C for 30 minutes.
It can survive in dried pus for 2-3 months.

Cell wall

Protein -A: It has a specific affinity for the Fc portion of the IgG molecule (except Ig3) leaving the Fab region free to combine with its specific antigen resulting in agglutination known as co-agglutination. The peptidoglycan of the cell activates complement and induces the release of inflammatory cytokines. Similarly teichoic acid of the cell wall facilitates adhesion of the cocci to the host cell surface.

Phage Types

With the use of 28 phages, several hundred phage types have been identified among them important phage types are-

Group 1: 8052A/79 (Hospital strains)
Group 2: 3B/3C/55 (Impetigo / Staphylococcal Scalded Syndrome)
Group 3: 6/47 (Enterotoxin producer)

Serotype

There are 30 serotypes based on protein A antigen.

Enzyme and Toxins

Toxins

Haemolysin: alpha, beta, gamma
Leukocidins
Enterotoxin A-F
Type A and B are responsible for food poisoning.
25 µg of toxin B can cause food poisoning.
Epidermolysins: It is responsible for Staphylococcal Scalded Syndrome (SSS) or Ritter’s disease.
Toxic Shock Syndrome Toxin (TSST): Type -F

Enzymes

Coagulase: It is of two types bound and free coagulase.

Phosphatase
DNAse

Staphylokinase
Hyaluronidase
Lipase
Protease

Pathogenicity of Staphylococcus aureus

Staphylococcus aureus can cause the following diseases-

Abscess
Conjunctivitis
Corneal ulcer
Septicemia
Endocarditis
Pneumonia
Mastitis: It is an inflammation of the breast.
Empyema: It is an accumulation of pus in the body cavity.
Food poisoning
Staphylococcal Scalded Syndrome
Toxic Shock Syndrome (TSS)-enterotoxin F
Septic arthritis
Meningitis
Osteomyelitis

Laboratory Diagnosis of Staphylococcus aureus

Samples/ specimens collection

It depends on the site of infection and the nature of the lesion. e.g.

Pus (Suppurative lesion)
CSF ( meningitis)
Blood (septicemia)
Sputum( respiratory infection)
Nasal swab (detection of carriers)
Feces and remains of food (food poisoning)

Gram stain: Gram-positive cocci in the cluster

Culture

Media -for routine Nutrient agar and blood agar

for Selective

7-10% salt agar
Mannitol salt agar

Tellurite glycine agar
Phenolphthalein phosphate agar
Polymyxin B agar (75 μg/ml)

Colony characteristics

on nutrient agar

Smooth, circular, often yellow-pigmented colonies and non-diffusible.

-1-2 mm in diameter

Butyrous inconsistency

On blood agar

Beta hemolytic

Biochemical tests

Catalase test: Positive

Oxidation and fermentation (OF) test: Fermentative

Coagulase test: Positive

DNAse test: Positive

From these features, the organism is identified as Staphylococcus aureus.

Coagulase test

Coagulase brings about the clotting of plasma which is similar to thrombin -catalytic conversion of fibrinogen into fibrin.

Types

Free coagulase: This is an extracellular enzyme of bacteria secreted into the medium. It is a thrombin-like substance that can change fibrinogen to fibrin. A tube coagulase test is performed for its detection.
Bound coagulase: This is closely bound to the cell wall. On its surface, it has receptors for fibrinogen so that fibrin forms links between the bacteria. This causes the clumping of Staphylococci. Hence, bound coagulase is also known as the clumping factor. Slide coagulase test is done for its detection.

Treatment

Following antibiotics are available for antibiotics sensitivity test (AST)-

Clindamycin
Erythromycin
Cefoxitin
Chloramphenicol
Ciprofloxacin
Gentamycin
Ofloxacin
Cotrimoxazole
Doxycycline
Vancomycin
Teicoplanin
Linezolid
Nitrofurantoin

Keynotes

Nitrofurantoin is only applicable in case of urinary tract infection replacing chloramphenicol.
To treat Methicillin-resistant Staphylococcus aureus (MRSA), vancomycin is recommended whereas to treat Vancomycin-resistant Staphylococcus aureus (VRSA), linezolid is preferred.

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