Coupon Accepted Successfully!



  • Family Micrococcaceae; Micrococcus (oxidase +, strict aerobe)
  • Family Staphylococcoses; Plano coccus (motile coccus), Staphylococcus

A. Staphylococcus:

  1. 33 defined species, 20 associated with colonization/ infection in humans
  2. of 20, one is coagulase positive, rest negative
  3. Infections with coagulase negative staphylococci (CNS) caused by S. epidermidis, S. saprophyticus, S. haemolyticus
  4. Gram positive, catalase positive, spherical cocci, divide incompletely in three planes forming clumps of    varying size, facultative anaerobes, G+C: 30-39 mol%
  5. First seen by Koch (1878), first cultivated by Pasteur (1880), named by Sir Alexander Ogston (1881); Staphyle means bunch of grapes

In a IV drug addicts with vegetation on tricuspid valve, most commonly association with which bacteria: (AIIMS Nov 09)
a) Klebsiella             
b) Staph aureus               
c) Pseudomonas               
d) E. coli

Ans- b. Staph aureus

B. Morphology

  1. 0.8-1.0micron, grape like clusters, nonmotile, nonsporing,
  2. noncapsulated (rare strains have microcapsule),GPC.    

C.  Cultural characteristics:

  1. Aerobes & facultative anaerobes, optimum temp. 37°C, optimum pH 7.5, nonfastidious
  2. Nutrient agar: 1-2mm, smooth, opaque, easily emulsifiable, golden yellow pigment (most strains), called staphylo xanthine.
  3. Pigment enhanced at 22°C; on milk agar
  4. Blood agar: beta hemolysis;
  5. MacConkey’s agar: a mildly selective medium for GN bacteria; tiny and pink coloured; Milk agar: larger & pigmentation enhanced; Phenolphthalein phosphate agar: produce phosphatase, colonies appear bright pink in colour;
  6. Selective media: Nutrient agar or milk agar with 7-10% salt, isolation from food, dust, faeces, pus; Enrichment medium: RCM with 10% salt; Mannitol salt agar: indicator & selective medium (1% mannitol, 7.5% salt, phenol red): colonies surrounded by yellow zones (due to mannitol fermentation)      

D.  Susceptibility to physical & chemical agents:-

chlorhexidine, phenol, crystal violet (1 in 500000)

  1. Initially sensitive to penicillin, but 1945 onwards penicillinase producing strains encountered (encoded by genes located on plasmid, transferred by transduction), another mechanism is due to reduction in affinity of penicillin binding proteins (chromosomally mediated MRSA). 

Other pigment producers- Pseudomonas, Serratia marcesans, Chromobacterium violaceum, Flavobacterium, Legionella pneumophila, Group B Streptococci


E.  Biochemical reaction:

Catalase +, Oxidase -, ferments glucose, maltose, lactose, sucrose & mannitol with the production of acid and gas; indole -, MR+, VP+, Urease+, gelatine liquefaction+, Nitrate reduction+, DNase, phosphatase+.


F.  Antigenic structure:

  1. Capsular polysaccharide: few strains capsulated(microcapsule), more virulent, polysaccharide, inhibits phagocytosis,  promotes adherence to prosthetic devices.
  2. Teichoic acid: ribitol teichoic acid are antigenic, (S. epidermidis contains glycerol teichoic acid); adherence to   mucosal surface.
  3. Protein A: found in 90% of strains, binds to IgG molecules through Fc receptors leaving Fab   sites free to combine with specific antigen (called as co-agglutination). Strain used in antigen detection-Cowan I
  4. Clumping factor (bound coagulase): reacts directly with fibrinogen in plasma, causing rapid cell agglutination. Slide coagulase test. 12% of free coagulase producing strains don’t produce clumping factor. 

G. Virulence factors of S. aureus:

  1. Cell-wall associated
    a.  Peptidoglycan, teichoic acid, protein A, clumping factor
  2. Extracellular factors:
    a.  Hemolytic toxins a, ß, d, ?;
    b.  leucocidin;
    c.   epidermolytic toxin;
    d.  enterotoxins;
    e.  toxic shock syndrome toxin
  3. Extracellular enzymes:
    1. Coagulase,
    2. staphylokinase,
    3. hyaluronidase,
    4. deoxyribonuclease,
    5. lipase, phospholipase, proteases

Cell-wall associated structures



Inhibits inflammatory response

Teichoic acid

Adherence to mucosal surfaces

Protein A

Reacts with Fc region of IgG

Clumping factor

Binds to fibrinogen

Extracellular factors


a hemolysin

-         Inactivated at 70°C, reactivated paradoxically at 100°C

-         Lyses rabbit, sheep and human red blood cells

-         Leucocidal, cytotoxic, dermonecrotic, lethal

ß hemolysin

-         Sphingomyelinase;Lyses sheep, but not human or rabbit red blood

      cells; Exhibits hot-cold phenomenon

? hemolysin

-         Lyses rabbit, sheep and human red blood cells

d hemolysin

-         Lyses rabbit, sheep and human red blood cells


-         Two components F & S

-         Damage polymorphonuclear leukocytes & macrophages

Epidermolytic toxin (Exfoliative toxin)

-         Mainly belong to phage group II

-         Two types

-         ET-A: 30,000 daltons, heat stable, chromosomal gene

-         ET-B: 29,500 daltons, heat labile, plasmid controlled


-         Produced by 40% of clinical isolates, heat stable(100C x 10-40min

-         Cause food poisoning; nine antigenic types (A,B,C,D,E,G,H,I,J)

Toxic shock syndrome toxin 1

-         Most strains producing it belong to phage group I

-         22,000 daltons

-         Earlier called as enterotoxin F


Extracellular enzymes


Free Coagulase

-         Activate coagulase reacting factor normally present in plasma

-         Clots plasma by converting fibrinogen to fibrin

-         Eight antigenic types (A-H)

Staphylokinase (fibrinolysin)

-         13,000-15,000 daltons

-         breaks fibrin clots & allows spread of infection


-         >90% strains produce

-         hydrolyses hyaluronic acid; facilitating spread of infection


-         Degrades DNA


-         Degrades lipid


-         Degrades phospholipids


-         Degrades proteins


H.  Pathogenicity

-    35-50% of normal adults carry S. aureus in anterior nares

  1. Cutaneous infections
    a. Wound, burn infections, pustules, furuncles, carbuncles, styes, impetigo
  2. Deep infections
    a.  Acute osteomyelitis, periostitis, tonsillitis, pharyngitis, sinusitis, bronchopneumonia, empyema,   septicaemia, meningitis, acute endocarditis, breast abscess, renal abscess.
  3. Exfoliative diseases
    a.  Due to strains producing exfoliative toxin.  
    b.  Cause intraepidermal blisters at the granular cell layer of epidermis
    c. These blisters range from mild pemphigus neonatorum to severe scaled skin syndrome (Ritter’s disease) in which toxin spreads systemically in individuals that lack neutralizing antibodies. It is characterized by extensive rash which sloughs off and skin surface resembles scalding
  4. Food poisoning
    a.  Occurs 2-6 hours after ingestion of food in which S. aureus has multiplied and formed enterotoxin,
    intradietetic. Meat, fish, milk & milk products are the food commonly implicated
  5. Toxic shock syndrome
    a.  Initially the disease was noted in healthy young menstruating women using highly absorbent tampons. Now however, non-menstrual cases are also as common.
    b.  Multisystem disease characterized by fever, rash, hypotension, vomiting, diarrhea, severe myalgias, disorientation, desquamation of skin (palms & soles).

I.   Laboratory diagnosis

  1. Specimens: Pus from suppurative lesions; blood from a case of PUO; sputum from a case of bronchopneumonia; mid-stream clean catch urine from a case of UTI; feces, food remains & vomit from a case of food poisoning.
  2. Direct examination: Gram stained smears are prepared from pus and wound exudates and are examined for presence of gram positive cocci in clusters and pus cells.     
  3. Culture: Specimens from sites containing other contaminating bacteria are inoculated on selective media (salt agar, salt milk agar).Inoculated media are incubated at overnight at 37°C. Next day plates are examined for typical colonies. Isolates are confirmed by various biochemical tests mentioned in the earlier section. 

J.   Bacteriophage typing:

  1. Phages of S. aureus have narrow host range. Therefore, for epidemiological studies, strains of S. aureus can be distinguished by their patterns of lysis by internationally recognized set of 23 standard typing phages. 

K. Treatment

  1. Antimicrobial agents that can be used are penicillins, cephalosporins, aminoglycosides, macrolides, lincosamides, glycopeptides, fluoroquinolones, rifampicin, fusidic acid, and carbapenems.
  2. Methicillin resistant staphylococcus aureus (MRSA): these strains are resistant to penicillinase resistant penicillins like cloxacillin, oxacillin, flucloxacillin & methicillin. Resistance is due to altered penicillin binding protein (PBP 2a), which has decreased affinity for these antibiotics.
    Resistance is probably arisen due to successive mutations and acquisition of resistance plasmid and is transmitted chromosomally due to mobile genetic element Staphylococcal Casette Chromosome mec.
    They are an important cause of nosocomial infections in debilitated patients.
    Glycopeptides (vancomycin & teicoplanin) are antibiotics of choice for treatment of systemic infections. Eradication of colonization sites (skin& nose) can be achieved using topical agents like mupirocin & chlorhexidine. 

Recent advances in MRSA-

  1. 2 types: Hospital acquired MRSA (HA-MRSA) and Community acquired MRSA (CA-MRSA). Risk factors for HA-MRSA: residence in an extended care facility, prior antibiotic exposure, insulin dependent diabetes, prolonged hospitalization, urinary catheterization, nasogastric tube placement, prior surgery. Cause bacteremia and UTI. Often MDR, esp. fluoroquinolones.
  2. CA-MRSA are MRSA isolated from patients with no h/o hospital exposure in past 1 yr. They have been found to be more virulent and cause skin and soft tissue infections, necrotising pneumonias and necrotising fasciitis. More than 90% strains produce the PVL toxin. They tend to be more susceptible to  antimicrobials.    

L.  Detection methods of MRSA-

  1. Agar dilution/broth dilution with Mueller Hinton agar/broth with 2-4% NaCl
  2. Kirby Bauer/Stokes disc diffusion method using 1microgram disc of Oxacillin on Mueller Hinton agar with 2-4% NaCl or 30microgram disc of cefoxitin on Mueller Hinton agar and incubation at 33-35C for 24 hrs
  3. E-test- strip based method using MHA with 2-4% NaCl
  4. Agar screen method using MHA with 2% NaCl and 6microgram Oxacillin
  5. Latex agglutination for detection of PBP2a
  6. PCR for mecA gene

Test Your Skills Now!
Take a Quiz now
Reviewer Name