Bacteriology- Staphylococci | Quick Revision notes for NEET-PG

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Gram-positive bacteria are divided on the basis of catalase test

  1. Staphylococcus species(catalase positive)
  2. Streptococcus species & Streptococcus-like species (catalase negative)

Staphylococcus species

Morphology- Gram-positive cocci arranged in single, pairs and grape-like clusters(due to division in 3 planes)

Non-motile, non-acid fast, facultative anaerobes, Catalase +ve & Oxidase -ve

Classification- Based on Coagulase test

1)Coagulase +ve – S. aureus

2)Coagulase – ve – S. epidermidis, S. saprophyticus, S. haemolytics

S. Aureus

 Culture-

  • Nutrient agar shows abundant pin-head golden-yellow colonies. This golden yellow pigment is k/a STAPHYLOXANTHIN which is carotenoid in nature.
  • 5% sheep blood agar shows beta hemolysis
  • MacConkey agar shows pink colonies indicative of Lactose fermentation
  • Selective medium
    • selective salt media(10% NaCl)
    • Mannitol Salt Agar(7.5% NaCl)-yellow discoloration around colonies
    • Ludlam’s tellurite medium.
  • Pigment enhancement media
    •  Milk agar(Milk +nutrient agar) enhance pigment better than salt milk agar(milk agar+7.5% salt)

Virulence factors

1)Cell wall factors like teichoic acid and Protein A. Protein A has a receptor for Fc part of IgG and resist complement activation making it anti-phagocytic. It forms the basis of the agglutination test.

the surface antigen of pneumococcus/gonococcus binds to the Fab part while the Fc part is attached to the protein A of S. Aureus.

2)Enzymes

  • Free coagulase ( it is the most important VF)- Coagulase +CRF/coagulase reacting factor activates prothrombin. Prothrombin converts fibrinogen to fibrin which clots forming a wall at the infected sites so, phagocytes cannot reach the organism leading to suppurative lesions/pyogenic infection. It is detected by the tube coagulase test. This enzyme can clot human /rabbit plasma but not guinea pig as CRF is absent in Guinea pig. Out of 8 types of Coagulase, Type A is most common.
  • Bound coagulase/clumping factor– activates Fibrinogen and inhibits complement-mediated opsonization. It is detected by the slide coagulase test.
  • DNAse
  • phosphatase
  • hyaluronidase
  • staphylokinase/fibrinolysin– 1)cleaves IgG and C3b and prevents opsonization 2)Breaks fibrin clots leading to spread of infection.
  • Lipase
  • Phospholipase
  • Proteases
  • Thermostable nuclease

3)Toxins-

  1. Hemolysins       
    1. alpha-hemolysin/paradoxical toxin-at 60-70 degrees toxin gets inactivated due to binding with heat labile inactivator and gets reactivated at 100 degrees. It lyses RBCs & leucocytes. It is dermonectrotic, neurotoxic and lethal.
    2. Beta hemolysin/sphingomyelinase/hot-cold hemolysis -breaks down sphingomyelin present in an increased concentration in sheep RBCs leading to hemolysis of sheep RBC. It exhibits Hot cold phenomenon i.e. lysis is initiated at 37o C but is evident only after cooling to 4o C since suspension occurs only when it is cooled down.
  2. Leucocidins/Panton-Valentine toxin-Made of 2 components F & S which synergistically create holes in WBC membrane and cause WBC destruction. This toxin is an example of -SYNERGOHYMENOTROPIC toxin
  1. Gamma toxin-It is a SynergoHymenoTropic toxin (together+membrane+stability) and it damages RBCs with no effect on WBCs.
  1. Epidermolytic toxin/exfoliatin/Extrafoliative toxin-superantigen produced by S. Aureus bacteriophage group 2. It acts on the skin and causes localized skin blisters, bullae formation, and separation of superficial skin layers. It causes staphylococcal scalded skin syndrome(SSSS) which is more severe and common in children k/a Ritter’s syndrome characterized by lethargy, poor feeding, irritation, and fever. Toxin is a serine protease that cleaves desmosomal cadherins. Positive for Nikolsky’s sign.
  1. Enterotoxins-It is pre-formed heat stable toxin and can withstand 1000C for 30 mins(secreted in food before consumption) which is expressed by nearly half of S. aureus strain and is responsible for Staphylococcal food poisoning. It acts rapidly by stimulation of vagal centers, vomiting center & intestinal peristalsis with a short incubation time of 1-6 hrs. The disease is characterized by nausea, vomiting, diarrhea, and dehydration with symptoms lasting for max. 10 hrs. It has 15 serotypes (A-E and G-P). It can be detected by ELISA/PCR/latex agglutination test
  1. Enterotoxin F/ Toxic shock syndrome toxin-it causes toxic shock syndrome. TSST-1 Gets absorbed into circulation from tampons(contaminated with S. Aureus)and then being a superantigen it stimulates T- cell by binding to T cell receptor causing excessive production and release of cytokines resulting in multiple organ failure. It can be detected by ELISA/PCR/latex agglutination test

DISEASES

Suppurative/Pyogenic                                                                                                                           

  • Bullous (SSSS)                                                                                                                                           
  • cellulitis   (inflammation of the skin and subcutaneous tissue)                                                       
  • Folliculitis (infection of hair follicles)                                                                                                
  • Furuncle
  • Carbuncle
  • Impetigo
  • Breast abscess(especially-lactating mothers)
  • Acute osteomyelitis
  • Infective endocarditis
  • Nosocomial pneumonia
  • Surgical site infection
  • Necrotizing pneumonia(by strains producing leucocidins)

Toxin Mediated

  • SSSS(Staphylococcal scalded skin disease)
  • TSS (Toxic shock syndrome)
  • Food poisoning

MRSA (Methicillin-resistant Staphylococcus aureus- community/hospital-acquired S. aureus) 

Back story- Penicillin G was the dug of choice if the organism was susceptible to it. Resistance to penicillin G developed due to the production of beta-lactamases by beta-lactamases coding gene(+nt in plasmid and transduced from R stain to S stain). After this failed, we made penicillin that was resistant to beta-lactamases eg methicillin, Nafcillin, Oxacillin etc. S. aureus developed resistance against these too and earned its name as MRSA. It is due to altered penicillin-binding protein (PBP-2a ) coded by mec A gene present on the bacterial chromosome.

PBP is an essential protein needed for cell wall synthesis. Beta-lactam drugs bind to it and inhibit this protein which in turns inhibit cell wall synthesis. Due to altered PBP protein i.e. PBP-2a, MRSA strains have less affinity for beta-lactam antibiotics, making these strains more resistant. 

Detection of MRSA can be done by

(1)antimicrobial susceptibility test using disc diffusion method using oxacillin/cefoxitin disc 

(2) PCR detecting mec A gene.

Drug of choice for MRSA is- Vancomycin. Other- 5th gen cephalosporins like ceftibuten and ceftobiprole.

Eventually, due to overuse of vancomycin, resistance towards vancomycin also developed.

Strains with low-grade resistance mediated by cell wall thickening is k/a VISA (vancomycin-intermediate S. Aureus ) and those of high-grade resistance medicated by Vac A gene are known as VRSA(Vancomycin Resistant Staphylococcus aureus).                                                     

DOC-Linezolid.

S. epidermidis

  • present as normal flora on skin oropharynx and vagina(commensals).
  • It is most common coagulase -ve Staph. Species.
  • It first adheres to prosthetic device surface and colonizes by producing biofilm(barrier) made of slime and glycan and multiplies within it protecting itself from host defense mechanism as well as antibiotics.
  • It causes endocarditis (with the insertion of shunts) and stitch abscess.

 

Fig1- Co-gglutinantion

Fig2- Tube colagulase Test

Fig3- Slide Coagulase Test

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