Medically Important Bacteria
Gram Positive Cocci
Clinically Important Species

Many species are medically important
1.
S. aureus – Coagulase + ve


2.
Most virulent species
Most common cause of bacterial infections, food
poisoning & toxic shock syndrome
Coagulase –ve Staphylococci

S. epidermidis – important cause of prosthetic

S. saprophyticus – Urinary Tract Infection in young
implant infections
women
Staphylococcus aureus – General features
Coagulase positive
 Beta hemolytic colonies
on BA
 Produces golden yellow
pigment
 Highly resistant bacteria
 Can grow in the presence
of 10 – 15% NaCl

Virulence Factors
1.
2.
3.
Toxins – cytolytic & superantigen
exotoxins
Enzymes &
Cell associated polymers and surface
proteins
1. Toxins
Haemolysins - Cytolytic, lyse RBCs of various animal
species
Leucocidins - Kills leucocytes
Enterotoxin A to E - Food poisoning
Toxic shock syndrome toxin (TSST/ Enterotoxin F) –
Toxic Shock Syndrome: rash, adesqumation, multi organ
failure
Epidermolytic ( Exfoliative ) toxin A & B –:
Staphylococcal scalded skin syndrome (epidermal
splitting & exfoliation)
2. Extracellular enzymes
Catalase – enhance their survival in phagocytes by
inactivating toxic H2O2 & free radicals released after the
ingestion of staphylococci.
Coagulase - Clots plasma, responsible for ‘tube coagulase
test’, confirmatory test for S. aureus
Hyaluronidase - Breaks down hyaluronic acid (connective
tissue): initiation & spread of infection
Fibrinolysin (Staphylokinase) - Lyses fibrin clots: spread of
infection
Nuclease - hydrolyses DNA
Lipase – Lipolytic: infection of skin & subcutaneous tissue
3. Surface proteins
Protein A - Antiphagocytic
Clumping factor - bound coagulase,
responsible for ‘slide coagulase test’,
screening test for S.aureus
Pathogenicity of S. aureus
Cutaneous infections –
– Folliculitis (boils), furuncle, burns and wounds
Deep infections –
– Osteomyelitis, abscesses, pneumonia, endocarditis,
septicemia
Toxin mediated infections –
– Staphylococcal scalded skin syndrome (SSSS),
– Toxic Shock Syndrome (TSS),
– Food poisoning (in 1-8hr, vomiting ,diarrhea,
nausea, self limited )
Pathogenicity
(Staphylococcal diseases)
Abscess
Toxic shock syndrome
Folliculitis
SSSS
Lab diagnosis – S. aureus
Specimens: wound swab,
pus, blood, feces
Microscopy: Gram stain GPC in clusters
Culture
– BA : beta hemolysis
– NA : golden yellow pigment
Catalase positive
Coagulase positive
Coagulase Test
Staphylococcus epidermidis
Major component of skin flora
Nosocomial infections: device/ implant associated
infections - shunts, catheters, artificial heart valves / joints,
pacemaker
Identification
BA: Non - hemolytic
Coagulase negative
Prosthetic valve endocarditis
Drug Resistance in Staphylococci
& MRSA
First developed resistance against Penicillin
Resistance to penicillin is mainly attributed to
the production of enzyme, penicillinase (betalactamase)
To combat resistance due to penicillinase,
Methicillin was developed & now methicillin
resistant strains have evolved – MRSA
MRSA
(Methicillin Resistant S. aureus)
Important cause of Nosocomial infections
– post surgical wound infections
– blood stream infections
– ventilator associated pneumonia
Patients with open wounds, invasive devices and
weakened immune systems are at greater risk for infection
Person to person spread – mainly from carriers (hospital
staff, visitors), 25-30% carry in their nose. By contact with
1. colonized or infected patients
2. colonized or infected body sites of the personnel themselves,
3. devices, items, or environmental surfaces contaminated with body
fluids containing MRSA.