8 – Streptococci and other Gram Positive Bacteria

Gram (+) Morphology and General Characteristics
– Gram (+), occurs in chains or pairs
– Catalase (-)
– Usually encapsulated
– Normal flora: Respiratory tract, genitourinary tract, gastrointestinal tract

Ways to classify Streptococci
– Serologically (Lancefield grouping)
– Hemolysis patterns
– Species (Biochemical patterns)

Lancefield Grouping
– Serological (A-W), Based on major cell wall carbohydrates
– Originally developed to differentiate Beta-hemolytic strains
– System inadequate
– Currently used to identify few *medically important strains* (A, B, C, D, F, G)

Medically Important Group A Streptococci
– Streptococcus pyogenes (strep throat and flesh eating bacteria)

Medically Important Group B Streptococci
– Streptococcus agalactiae (cause neonatal sepsis)

Medically Important Group C and G Streptococci
– S. milleri
– S. dysgalactiae
(small percentage of strep throat are caused by these)

Medically Important Group D Streptococci
– Enterococci: E. faecalis and E. faecium
– Nonenterococcal Group D: S. bovis
→ endocarditis associated with gastrointestinal neoplasms

Medically Important Streptococci: Viridans Streptococci
(endocarditis, dental caries, etc.)
– Heterogenous group
– Several species: S. salivarius, S. mitis, S. sanquis, and S. mutans

Classification by hemolysis
Hemolysis exhibited on blood agar (media contains 5% sheep RBCs)
– alpha (α) – incomplete hemolysis, colony surrounded by a green halo
– beta (β) – complete hemolysis, colony surrounded by a clear zone
– gamma (γ) – no hemolysis (non-hemolytic)

Streptococcus pyogenes
– (GAS= group A strep)
-β -hemolytic (GABHS) – group A beta hemolytic strep
– Bacitracin sensitive
– M protein
→ 100 antigenic types
→ PCR now used to assign types to GAS strains
→ Antibodies are protective against re-infection with the same type but not different types
→ Antibodies to M proteins can lead to autoimmune disease
→Possesses epitopes immunologically similar to human epitopes → ARF, PSGN (post strep glomerular nephritis.

Virulence Factors:
– structural components
– extracellular products

Virulence Factors: Structural components
– *M protein*: blocks C3b binding
– *M-like proteins*: bind Fc portion of antibody
– *Hyaluronic acid capsule*: weak immunogen; antiphagocytic (makes organism look like host)
– Forms biofilms
– Adhesion proteins: LTA, M protein, F protein (allow them to survive innate immune system attack)
– Invasion of epithelial cells: M and F protein

Virulence Factors: Extracellular products
– Spreading factors:Streptokinase, DNase, protease
– Streptolysin S and O: damage cell membranes & accounts for hemolysis
– Streptococcal pyrogenic exotoxins (Spe A, B, C)

Streptococcal pyrogenic exotoxins
(Spe A, B, C)
– Produced by lysogenic strains (expressed as prohage); diphtheria toxin
– Act as *superantigens*
-Responsible for rash associated with scarlet fever
– Linked to severe invasive infections

Suppurative Manifestations
Produce pus
– Pharyngitis
– Impetigo
– Erysipelas
– Necrotizing fascitis

(Suppurative Manifestations)
– Strep pyogenes
→ most common bacterial cause of exudative pharyngitis in children
– Nonsuppurative complications: ARF and PSGN
– Scarlet fever exanthem (rash) associated w/expression of Spe A, B, or C (exotoxins).

(Suppurative Manifestations)
– associated with strep pyogenes
– (pyoderma): superficial infection of the skin

(Suppurative Manifestations)
– associated with strep pyrogenes
– form of cellulitis, involves skin & subcutaneous tissue and includes systemic signs (usually preceded by resp. or skin infection w/S. pyogenes)

Necrotizing fascitis
(Suppurative Manifestations)
– associated with strep progenies
– destruction of muscle and fat
streptococcal gangrene (necrosis & obstructed blood flow)
– Hallmarks: toxicity, multiorgan failure (because they are superantigens), death
– Strep implicated in 60% of necrotizing fasciitis cases

Streptococcal Toxic Shock Syndrome
– Associated w/strains producing: Spe A or C; M serotypes 1 or 3; prominent capsule (mucoid strains)
– Freq. associate w/necrotizing fasciitis & bacteremia
– Criteria available for diagnosis
– Aggressive medical intervention indicated; >30% of cases are fatal

Non-Suppurative Complications (Sequelae)
– Acute Rheumatic Fever (ARF)
– Post-Streptococcal Glomerulonephritis (PSGN)

Acute Rheumatic Fever (ARF)
(Non-Suppurative Complications)
– A multisystem disease resulting from an autoimmune reaction to infection with Group A strep.
– Cross-reactivity of streptococcal antigens and human heart, joint and nervous tissues
– Follows respiratory strep infections, but not skin infections
– Characterized by a pancarditis (inflammation of the ENTIRE heart) and arthritis

Post-Streptococcal Glomerulonephritis (PSGN)
(Non-Suppurative Complications)
– An autoimmune complex disease
– Antigen-antibody complexes deposited in the kidneys fix complement and damage glomeruli
– May follow respiratory or skin infection
– Acute inflammation of renal glomeruli w/edema, dark urine, and hypertension

Diagnosing Strep Pyogenes
– Gram (+)
– Culture on blood agar
– Sensitive to bacitracin (differentiates from Group B strep)
– Antibody to Streptolysin-O (ASO test) = + PYR (separates pyogenes from other strep)

Streptococcus agalactiae
– Group B strep
– Colonize the lower GI and GU tract; transient vaginal carriage
– A leading cause of *neonatal sepsis*, pneumonia and meningitis
– Early onset (endogenous): bacteremia, pneumonia or meningitis
– Late onset (exogenous): bacteremia with meningitis
– Antiphagocytic polysaccharide capsule is a major virulence factor

Clinical Considerations of Streptococcus agalactiae
– Colonizes female genitourinary tract
→ All pregnant women screened for colonization
→ 60% of infants born to colonized mothers become colonized
– Pregnant women
→ Postpartum endometritis, wound infections, UTI’s
– Non pregnant women and men
→ Opportunistic infections (HIV, diabetics, cancer): skin and soft tissue infections, bacteremia, urosepsis

Diagnosing Streptococcus agalactiae
– Gram stain of CSF
– PCR – based amplification assay

– group D strep (GDS)
– Gram positive cocci, usually in pairs
– Variable hemolysis
– good example of opportunistic pathogen (seen in healthcare associated environment, UTIs)
– Normal intestinal flora (as name implies)
– Virulence: resistance to commonly used antibiotics (VRE)
– One of the most common causes of nosocomial infections

Clinical Manifestations of Enterococci
– Urinary tract infections
– Nosocomial bacteremia
→ Associated with localized infection or endocarditis
– Bacterial endocarditis = commonly associated
→ 10-20% of cases on both native and prosthetic valves
– Peritonitis
→ Typically polymicrobial

Laboratory Identification (Testing) for Enterococci
– Growth in 6.5% salt
– Grows at 42°C
– Grows in the presence of 40% bile salts
– PYR-test: produces L-pyrrolidonyl-arylamidase (only S. pyogenes is +) like strep p but this one can be separated by growing in high salt

Treatment for Enterococci
– Synergistic combination antibiotic therapy (aminoglycoside and a cell wall active antibiotic)
– Can use Vancomycin, however VRE exist (resistance IS a problem with this organism)

Viridans Streptococci
– Heterogeneous collection of alpha and non-hemolytic strep
– Constitute main facultative oral flora
– Contribute to dental caries
– Bacteremia in neutropenic patients
– *Sub-acute bacterial endocarditis* in patients with abnormal heart valves, dental caries, and intra-abdominal infections

Streptococcus pneumonia
– important cause of CAP
-Gram positive diplococci, lancet shaped in short chains (important diagnostically)
Alpha-hemolytic on blood agar
– Optochin sensitive (chemical), lysed by bile
– Encapsulated, 90 serotypes, most serotypes included in polyvalent vaccine

Streptococcus pneumonia: Virulence Factors
– Capsule
– Surface protein adhesions
– Release of cell wall components → inflammatory response
– Pneumolysin
→ exotoxin that creates pores in ciliated epithelial cells and phagocytes; activates complement → migration of inflammatory cells → tissue damage
– Secretory IgA protease

Streptococcus pneumonia: Epidemiology/Pathogenesis
– Normal flora of throat and nasopharynx, infection can be endogenous or exogenous
– Highest incidence in children and the elderly because of low levels of protective antibodies

Streptococcus pneumonia: Clinical Disease
– Typical lobar pneumonia: Leading cause
– Meningitis: Most common cause in children and young adults.
– Otitis media
– Sinusitis
– Bacteremia

Streptococcus pneumonia: Laboratory Diagnosis
– Lancet-shaped diplococcus
– Optochin sensitive
– Bile soluble (dissolves in bile)
– Quellung reaction to detect capsule (mix with antibodies specific for __, capsule will swell)
– Pneumococcal C polysaccharide can be detected in urine

Streptococcus pneumonia: Treatment and Prevention
– Antibiotics: will be discussed in systems
– Polyvalent vaccines for young children and for 55↑ available

Gram (+) Cocci flow chart
Gram (+) Cocci flow chart

Another Gram (+) Cocci flow chart
Another Gram (+) Cocci flow chart

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