streptococcus Flashcards
streptococcus characteristics
gram-positive cocci; typically arranged in chains or pairs
catalase negative - this trait distinguishes them from staphylococcus species, which are catalase-positive; if catalase is present, the enzymatic reaction can be witnessed as bubbles of O2 accumulate
streptococcus classification
test performed on blood agar plates
beta-hemolysis
bacteria that are capable of beta-hemolysis produce exotoxins that deteriorate RBCs causing complete lysis
alpha-hemolysis
bacteria that are capable of alpha-hemolysis secrete hydrogen peroxide which causes oxidation of hemoglobin, turning the medium green
gamma-hemolysis
describes bacteria that do not induce hemolysis of RBCs
lancefield groups
serological classification scheme developed by Rebecca Lancefield for differentiationg species of Streptococcus; identifies species of Streptococcus by unique carbohydrate antigens in their cell wall; most important medically are Group A and Group B streptococci
Streptococcus pyogenes
the only species of GAS; S. pyogenes is beta-hemolytic
Streptococcus pyogenes epidemiology
the bacteria transiently colonize the upper respiratory tract and skin; bacteria are spread between hosts through respiratory droplets or through direct contact with breaks in the skin
Streptococcus pyogenes virulence factors (6 factors)
capsule - avoids phagocytosis
M protein - prevents complement-mediated phagocytosis; adheres to host cells; allows for invasion into host epithelial cells
C5a peptidase - inactivates C5a preventing attraction of phagocytes
Streptococcal pyrogenic exotoxins (Spe family) - this family of exotoxins act as superantigens that interact with macrophages and helper T cells causing the enhanced release of proinflammatory cytokines; responsible for the clinical manifestations of severe streptococcal diseases
streptolysins - exotoxins that lyse erythrocytes, leukocyts, and platelets (reason for beta-hemolysis on blood agar)
streptokinases - cleave fibrin and fibrinogen resulting in the lysis of clots allowing the bacteria to spread
strepotcoccus pyogenes diseases
pharyngitis (strep throat)
scarlet fever - a complication of pharyngitis that occurs when the infecting strain produces a pyrogenic exotoxin (Spe); diffuse rash appears; strawberry tongue
pyoderma (impetigo)
erysipelas (red skin)
cellulitis
necrotizing fasciitis (streptococcal gangrene)
bacteremia - strongly fcorrespond with necrotizing fasciitis
streptoccocal toxic shock syndrome - due to production of pyrogenic exotoxins (Spe)
rheumatic fever - complication of pharyngitis
acute glomerulonephritis
group b streptococci (GBS)
streptococcus agalactiae only species of GBS; beta-hemolytic
s. agalactiae epidemiology
colonize the lower gastrointestinal tract and the genitourinary tract; transient vaginal carriage is observed; primarily a pathogen of neonates
s. agalactiae virulence factors (two)
capsule
streptolysins - exotoxins that lyse erythrocytes, leukocytes, and platelets (reason for beta-hemolysis on blood agar)
s. agalactiae pathogenesis
the capsule prevents phagocytosis until specific antibodies develop; the lack of s. agalactiae-specific antibodies explains the preference for neonates; the calssical and alternative complement pathways are required for killing s. agalactiae during early infection; therefore there is a greater likelihood of systemic spread in premature babies who have low complement levels
s. agalactiae disease
neonatal disease - bacteria are acquired in utero, at birht, or shortly after birth; disease is characterized by bacteremia, pneumonia or meningitis
infections in pregnant women - postpartum endometritis, wound infection, and urinary tract infections can occur during and immediately after pregnancy
viridians streptococci
the majority of streococci in this group are alpha-hemolytic on blood agar
s. pneumoniae
the most virulent member of the viridans streptococci
displays alpha-hemolysis when grown aerobically
s. pneumoniae epidemiology
common inhabitant of the nasopharynx and oropharynx in healthy people
s. pneumoniae virulence factors (three)
capsule - avoids phagocytosis; primary mechanism of virulence during initial infection; if a strain loses the ability to make the capsule, the strain is no longer virulent
pneumolysin - a cytotoxin that binds to cholesterol in host cell membranes and causes pores to form; this activity destroys ciliated epithelial cells and phagocytic cells
IgA protease - inactivates IgA
s. pneumoniae pathogenesis
bacteria residing in the nasopharynx or oropharynx penetrate the epithelial layer and then gain access to other body areas such as the lungs, sinuses or middle ear; if the bacteria gain access to the blood, the meninges may become infected
s. pneumoniae diseases
pneumonia
meningitis
bacteremia
