Gram-positive cocc
-1 μm diameter
-grape-like clusters
- important for humans
examples:
Staphylococcus epidermidis - normal flora of skin and mucosae; occasionally pathogenic
Staphylococcus aureus - pathogenic; often found as normal flora
Staphylococcus saprophyticus - in the environment and on skin; can cause urinary infections
Staphylococcus aureus
-simple media (agar, blood-agar)
- 2-3 mm colonies in 24 h (37ºC)
-toxic to many cells: leukocytes, erythrocytes, macrophages, platelets and fibroblasts
-important cause of food poisoning (nausea, vomiting, cramps, diarrheas)
- “scalded skin” syndrome in infants
- cause of toxic shock syndrome
-food poisoning
- coagulase test is used for the identification of Staphylococcus aureus
-destroys penicillin
- widely spread in the normal population
-15% of healthy individuals may
carry it at one time or another in the anterior nares, axilla, perineum, etc. and on hands.
- produce localized purulent infections (pustules, boils, styes, conjunctivitis,
otitis, etc)
complications: pneumonia (diabetics, leukemia/lymphoma, steroid
therapy)
- bacteriophage types” as identity markers in tracing sources of infection.
Preventive measures against staphylococcal nosocomial infections include:
- Strict adherence to aseptic techniques in the operating rooms and to wound precaution techniques
in handling post-operative infections - Education of hospital personnel in the mode of transmission (mainly by contact) of staphylococcal
infections and in the paramount importance of handwashing.
Staphylococcus epidermidis
- part of normal skin/mucous membranes flora
-non-pathogenic - cause infections such as post-operative infections after brain or open-heart surgery, endocarditis after prosthetic heart valve insertion, shunt infections, etc.
STREPTOCOCCI
- Gram-positive cocci usually arranged in pairs or forming chains.
- Fastidious in their nutritional requirements (grown on blood agar)
-3 subdivisions
Gram-positive cocci subdivided in 3 parts
Haemolytic properties: alpha-haemolysis, greenish brown zone of partial red blood cell destruction
beta-haemolysis, clear zone of complete red blood cell destruction around
colonies
some strains are non-hemolytic
b) Carbohydrate C-antigen: extracted from the cell wall, subdivides streptococci in groups A-T (Lancefield
classification).
c) M-protein: Permits subdivision of beta-haemolytic streptococci into over 70 serotypes;
found almost exclusively in group A. Important virulence factor: antiphagocytic
and degrades complement C3b
Streptococcus pyogenes
Acute tonsillitis - (“strep sore throat”) - complications can result in Scarlet Fever
Skin infections - cellulitis, erysipelas, wound/burn infections.
Puerperal fever (sepsis) - post-partum or post-abortion
complication in young patients:
(i)acute glomerulonephritis; and
(2) rheumatic fever involving joints and heart valves. S. pyogenes produces toxins and enzymes related to
virulence.
toxins: Streptolysins(O and S), beta-haemolytic and highly toxic for neutrophils and macrophages. An
anti-streptolysin-O (ASO) antibody reaches high titers after recent infections (EXCEPT in skin
infections).
Enzyme: Hyaluronidase - splits hyaluronic acid in connective tissue and helps spreading.
-found in 5-l0% of healthy individuals
-are sensitive to
penicillin G and so it is the antibiotic of choice (note penicillin sensitivities)
-Transmission is by direct contact (particullarly nasal carriers)
Prevention: education of health personnel and public on modes of transmission, strict
asepsis in obstetric procedures and early detection and treatment
STREPTOCOCCUS GROUP B (Streptococcus agalactiae)
frequently found in the vagina of healthy women, can cause neonatal infections
- Early septicaemia - respiratory distress/shock at birth or within 24 hours; high fatality rate
- Delayed meningitic form - between l-l2 weeks post-partum; prognosis considerably better, but
neurologic/mental abnormalities may result.
STREPTOCOCCUS GROUP D (Streptococcus faecalis, Enterococcus)
normal flora of the human gastrointestinal tract
- May cause infections when introduced into tissues: urinary tract infection
-Often resistant to a wide range of antibiotics.
VIRIDANS STREPTOCOCCI-
includes a group of different species of streptococci
- found in the oral cavity (mouth) of healthy individuals
-May cause endocarditis in individuals with previously damaged heart valves.
PNEUMOCOCCUS
-Streptococcus pneumoniae are diplococci (pairs) with a polysaccharide capsule having antiphagocytic properties.
-90 distinct capsular serotypes
- commonly found in the naso-pharynx of healthy individuals
-can cause lobar pneumonia and meningitis:
Lobar pneumonia - most often an auto-infection, though it may spread as a “carrier epidemic”. More frequent in infancy, old age and alcoholics.
Meningitis more often in infants and in the elderly and is accompanied by bacteraemia.
Prevention includes (l) programs for the elderly and alcoholics (2) avoidance of crowded living quarters and
(3) vaccination of those at highest risk with a polyvalent pneumococcal vaccine containing 23 capsular
polysaccharide serotypes
Meningitis
- purulent, with high fatality rate if not treated promptly
Septicaemia
high fever, rash that ranges from a few petechiae to haemorrhagic
rash with endotoxic shock.
Waterhouse-Friderichsen syndrome
bilateral adrenal cortical haemorrhage with fulminating
collapse/death in less than 24 hours. It is a complication of meningococcal septicaemia
Prevention and Treatment of neisseria meningitidis
- Conjugated vaccine that protects against serogroups A, C, Y and W135
- Vaccination is recommended for children 11-12 years, teenagers entering high school and college freshman
residing in dormitories. - Penicillin is the primary antibiotic used to treat infections; chloramphenicol or 3rd generation cephalosporins
can also be used.
