microbio

• Staphylococci


  Gram-positive spherical cells, arranged in grape-like irregular clusters
• Staphylococci


  • Some are members of the normal flora of the skin and mucous membranes of humans
  • Others cause suppuration, abscess formation, a variety of pyogenic infections, and even fatal septicemia
• Staphylococci


  • Non-motile
  • Non spore forming
  • Some strains posses capsule
  • Able to grow in a variety of conditions: aerobically & anaerobically, in the presence of a high concentration of salt, temperatures ranging from (18°c -40°c)
• Staphylococci


  • They are active metabolically, fermenting carbohydrates and producing pigments that vary from white to deep yellow
  • The pathogenic staphylococci often hemolyze blood, coagulate plasma, and produce a variety of extracellular enzymes and toxins
  • Staphylococci rapidly develop resistance to many antimicrobial agents and present difficult therapeutic problems
• Classification of Staphylococci


  • Coagulase positive staph
  • Coagulase negative staph (CoNS)
• Coagulase positive staph


  Eg. Staph. aureus
• Coagulase negative staph (CoNS)


  Eg. Staph. epidermidis, Staph. saprophyticus, Staph. haemolyticus
• Staphylococcus aureus


  • Gram positive cocci
  • Non motile
  • Non spore forming
  • Facultative anaerobe
  • It is both normal flora (nasal passages, Skin &M.M) and a human pathogen (wide range of infections)
• Virulence factors of Staph.aureus

  • Structural components: Capsule, Slime layer, Peptidoglycan, Teichoic acid, Protein A
  • Toxins: Cytotoxins, Exfoliative toxins (ETA, ETB), Enterotoxins (A-R), Toxic shock syndrome toxin-1, Hemolysin
  • Enzymes: Coagulase, Hyaluronidase, Fibrinolysin, Lipases, Nucleases
• Diseases caused by staph .aureus

  • Toxic-Mediated Diseases: Scalded Skin Syndrome, Food Poisoning, Toxic shock Syndrome
  • Suppurative infections: Impetigo, Folliculitis, Furuncles, Carbuncles, Bacteremia & endocarditis, Pneumonia & empyema, Osteomyelitis, Septic arthritis
• Suppurative infections


  • Carbuncles
  • Impetigo
  • Folliculitis
• Methicillin-resistant Staphylococcus aureus (MRSA)


  A strain of Staphylococcus aureus that acquired resistance to methicillin and other beta lactam antibiotics (e.g., penicillins and cephalosporins) via genes (eg, mecA, mecC)
• How MRSA spreads


  • Hands of health care providers
  • Having direct contact with another person's infection
  • Sharing personal items (towels, razors) that have touched infected skin
  • Touching surfaces or items, (eg, bandages) contaminated with MRSA
• Proper hand hygiene is the key to preventing the spread of MRSA
• Types of MRSA infection

  • Hospital (Nosocomial) acquired MRSA: MRSA acquired after 48hrs stay in hospital with no previous documentation of a positive MRSA culture, Risk factors: extensive antibiotic therapy, admission in ICU, central venous catheter, long duration hospital stay, Toxin production: rare, Presentation: bacteremia, pneumonia, Highly drug resistant
  • Community acquired MRSA: MRSA acquired in the community, Toxin production: common, Presentation: asymptomatic colonization, SSTI, bacteremia, pneumonia, Less drug resistant
• Prevention of MRSA


  • Hand washing
  • Keep opened wound covered
  • Avoid sharing personal items
  • Report/seek medical advice if they suffer from wound, pimple or boil that does not heal or getting worse
• Coagulase Negative Staph Aureus (CoNS)


  Usually inhabits skin & m.membrane, are common contaminants in clinical specimens, Frequently associated with nosocomial infection
• Patients at risk for CoNS infection

  • Those with prosthetic devices
  • Pacemakers
  • Intravascular catheters
  • Immunocompromised hosts
• Staph epidermidis


  • It is part of normal flora (skin, MM), It is true opportunistic pathogen, Accounting for approximately 60-70% of all coagulase-negative Staphylococci on the skin
  • Virulence Factors: Exopolysaccharide (Slime or biofilm), Exotoxins: Delta toxin
• Staph saprophyticus


  • It is part of normal vaginal flora, Predominantly implicated in genitourinary tract infection (sexually active young female), 2nd most common cause of UTI, after E.coli in young women, It also causes soft tissue infection
  • Coagulase & phosphatase: negative, Lipase & urease: positive, Resistant to the antibiotic Novobiocin (differentiates it from Staph.epidermidids)
• Diseases caused by CoNS

  • Bacteremia
  • Intravascular catheter infection
  • Bacterial endocarditis
  • Urinary tract infection
  • Osteomyelitis
  • Other infections: prosthetic valves, cerebrospinal fluid shunts, infections of medical devices, cosmetic implants
• Laboratory Diagnosis of Staphylococci

  • Smears (Direct film): Gram positive cocci arranged in clusters
  • Culture: Specimens planted on blood agar plates give rise to typical colonies in 18 hours at 37 °C, Media used: Blood agar, chocolate agar, MacConkey agar, Mannitol salt agar, Glucose broth, brain-heart infusion broth
  • Biochemical tests: Catalase test, Coagulase test, DNase test, Novobiocin Susceptibility test
  • API Staph test: Combination of standard biochemical tests and fermentation tests
  • Rapid diagnostic tests: Real-time PCR
• Colony morphology on agar plates

  • On blood agar: Staph.aureus - golden colonies usually seen & frequently surrounded by clear zone of B- hemolysis, Staph.epidermidis - white - creamy colonies, No hemolysis, Staph.saprophyticus - White-yellow colonies, No hemolysis
  • On mannitol salt agar: Staph.aureus - produces yellow colonies with yellow zones surrounding (ferment mannitol), Other staph - produces small pink / red colonies and they do not change the color of the medium (non-mannitol fermenter)
  • On nutrient agar: Staph.aureus - circular, smooth, shiny surface & are pigmented (golden-yellow)
• Biochemical tests


  • Catalase test: Used to differentiate between Staph (Catalase positive) and Strept (Catalase negative)
  • Coagulase test: Used to differentiate staph. aureus from Coagulase- negative staph
  • DNase test: Positive - clear zone around the spot or streak indicates DNase activity
  • Novobiocin Susceptibility test
• API Staph test

  Combination of standard biochemical tests and fermentation tests which are the reference tests for the identification of staph
• Rapid diagnostic tests

  Techniques include Real-time PCR which is increasingly being used in clinical laboratory
• Factors affecting the type of treatment chosen

  • The location, severity and progression of the infection
  • The age and health of the patient
• Skin infections are sometimes treated with techniques that do not require systemic antibiotics (e.g., incision and drainage of abscesses)
• Treatment may also require adjunct measures such as the removal of catheters
• Antibiotics must be selected based on susceptibility testing