Gram-positive cocci
Bacteria
Gram positive bacteria Gram negative bacteria
1-Gram Positive bacteria
Two pathogenic bacteria are G+ve:
1-Staphylococci.
2- Streptococci.
Staphylococci
I- General features:
1- Staphylococci are G+ve cocci (spherical or grapes shape).
2- Staphylococci are non motile, non capsulated, non spore
forming.
3- Staphylococci are oxidase negative & catalase positive which
one feature that distinguishes from Streptococci.
4- Staphylococci are part of normal flora of human skin, nose,
respiratory and gastrointestinal tracts.
Are also found in air, dust and other in human environments.
5- Staphylococci has at least 30 spp., three spp of clinical
importance are Staphylococcus aureus (S. pyogenes),
S. epidermidis (S. albus), S. saprophyticus (S. citrus).
II- Transmission:
S. aureus is major pathogenic spp for human.
Transmission of bacteria from human to human by inhalation of
respiratory secretion or consumption of contaminated food.
III- Pathogencity & clinical features:
A: Pathogenesis
Pathogenic virulence factors are enable an organism to produce
disease. The clinical outcome of an infection depends on the
virulence of the pathogen and the opposing effectiveness of the
host defense mechanisms.
Pathogenesis of S. aureus depends on the combined actions of
several virulence factors, so it is difficult to determine precisely the
role of any given factor.
S. aureus expresses many potential virulence factors:
1- Cell wall virulence factors (Protein A and protein F).
2- Cytolytic exotoxins or hemolysins (α, β, γ, δ toxin).
3- Superantigen exotoxins: {Enterotoxins (six major antigenic types:
A, B, C, D, E, and G) and Toxic shock syndrome toxin (TSST-1)}
B: Clinical significance
Staphylococcal infections are classified as:
1-Skin infections: such as abscess, pyoderma (impetigo),
furuncles, carbuncles, styes, boils, folliculitis, cellulites, toxic shock
syndrome, and scalded skin syndrom.
2-Respiratory tract infections: such as tonsillitis, pharyngitis,
sinusitis, pneumonia, and Otitis media.
3-Other infections: endocarditis, osteomyelitis, meningitis,
and nosocomial infections.
4-Food poisoning (Staphylococcal gasteroenteritis) :is due to
enterotoxin, which characterized by short incubation period
(1-6 hr.) after consumption contaminated food. Vomiting, diarrhea,
nausea, never fever are most symptoms.
IV- Laboratory diagnosis:
A: smear examination: stained smear shows G+ve cocci
arranged in cluster.
B. culture of S. aureus: the sample is plated on blood agar,
showing yellow colonies with Beta hemolytic.
Identifications of bacteria is confirmed by catalase positive,
coagulase test positive, mannitol fermentation, and grow in high
concentration (7.5%) of NaCl.
V- Control
Treatment: oxacillin, vancomycin, cephalosporins, and ciprofloxacin
2. Streptococci
I- General features:
1. Streptococci are G+ve cocci (spherical,
chain or Pairs shape) .
2. Are non motile, non spore forming and
non capsulated (some strain have capsule).
3. Streptococci are oxidase & catalase
negative which one feature that distinguishes
the Streptococci from Staphylococci.
4. Streptococci are member of normal flora skin,
respiratory tract and some are normal flora of enteric and genital
tracts of human.
5. Streptococci has at least 20 spp. S. pyogenes, and S.
pneumoniae are clinical Importance for human.
II- Transmission:
Respiratory tract infections (S. pyogenes) are transmitted by
inhalation of respiratory droplets. Skin infection occurs after direct
contact with infected individuals or contaminated fomites.
III- Pathogencity & clinical features:
A: Pathogenesis
S. pyogenes cells, perhaps in an inhaled droplet, attach to the
pharyngeal mucosa via actions of protein F and M protein. The
bacteria may simply colonize.
Alternatively, bacteria may grow and secrete toxins, causing
damage to surrounding cells, invading the mucosa, and eliciting
an inflammatory response with attendant influx of white cells, fluid
leakage, and pus formation. The patient then has streptococcal
pharyngitis.
There is sufficient spread that the bloodstream is significantly
invaded, possibly resulting in septicemia and/or seeding of distant
sites, where
cellulitis (acute inflammation of subcutaneous tissue),
fasciitis (inflammation of the tissue under the skin that covers
a surface of underlying tissue), or
myonecrosis (death of muscle cells) may develop rapidly or
insidiously.
S. pyogenes expresses many potential virulence factors:
1- Capsule (hyalouronic acid).
2- Cell wall virulence factors (Protein M, and protein F).
3- Extracellular products; hemolysins or (streptolysin O, S),
streptokinase, streptodoranses, and hyaluronidase.
4- Superantigen exotoxins: { Streptococcal shock syndrome toxin
(SSST)}.
B: Clinical features Streptococcal infections
Streptococcal infection are are classified as:
1-Pyogenic infections (skin & respiratory tract infection)
Skin infections:
A: Erysipelas: red area has rapid spread to give a butter fly
distribution with blister on skin of face.
B: Puerperal sepsis: this infection is following the delivery of
newborn.
C: Cellulitus: erythema, swelling, pain lesion of skin and
subcutaneos tissues. The infection is associated with burns,
wound or surgical incisions.
Respiratory tract infection:
A: Sore throat (tonsillitis) after incubation periods (1-3 days), or
its may be invade pharynx and causes pharyngitis.
B: It may be causes severe pneumonia with fever and cough.
2-Toxigenic infections:
Scarlet fever: it is caused by ethrogenic toxin; the lesion is
associated with pharyngitis or skin or soft tissues infection. The
lesion is characterized by fever, vomiting, rash, peeling of skin, and
straw berry tongue. The rash appears on trunk after 24hr. of
illness.
3-Immunogenic disorder: the infections occur after (1-3 weeks)
from acute infections.
A: Rheumatic fever (RF): is occurs after upper respiratory tract
infection (URTI) with sore throat not after skin infection. RF is
caused by autoantibodies that react with heart muscle due to
similarity in structure.
B: Acute Glumerulonephritis (AGN): is one of most common
complications of URTI and skin infection. AGN is immune complex
disease, which lead damage of kidney.
IV- Laboratory diagnosis:
A: Microscopic smear: smear shows G+ve cocci arranged in
chines or pairs.
B. culture of S. pyogenes: the sample is plated on blood agar,
showing small colonies with Beta hemolytic. Identifications of S.
pyogenes is confirmed by Bacitracin sensitivity test.
C: Serological test: are a rise in titer of antibodies against group A
streptococci can be estimated such as antistreptolysin-O (ASO), if
ASO titer in streptococci serum is excess of 160-200 units is
indicate streptococcal infection especially RF.
V- Control
Treatment: penicillin G, aminoglycoside, erthromycin and
ciprofloxacin. Antimicrobial drug have no effect on AGN and RF
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