Staph, Strep, and Related Gram-Positive Cocci

Catalase-positive cocci

• Staphylococcus spp.

• Micrococcus

• Rothia

What are Staphylococcal exotoxins responsible for?

Food poisoning

Scalded skin syndrome

Toxic shock syndrome

What differentiates Staphylococcus species?

S. aureus is coagulase ( + )

• DNAse ( + )

• Mannitol ( + )

S. epidermidis and S. saprophyticus are coagulase ( - )

Sites of colonization of S. aureus

• Anterior nares

• Skin folds

• Perineum

• Vagina

• Armpits

Rates as high as 60%

Predisposing factors to S. aureus infection

• Leukocyte chemotaxis defects: Wiskott-Aldrich, diabetes, rheumatoid arthritis.

• Hypogammaglobulinemia, complement deficiencies.

• Absence of peroxides or superoxide from phagocytic vacuoles: chronic granulomatous disease.

• Skin injuries

• Foreign bodies

• Infection with other agents, like viruses

• Chronic underlying disease, alcoholism.

• Antibiotics

Protein A

• S. aureus virulence factor; used to ID

• Binds to the Fc region of IgG to prevent opsonization

• Activates complement and elicits hypersensitivity rxns

• Promotes adherence of bacteria to platelets (vWF)

Superantigens

The ability of toxins to stimulate the proliferation of T-cells without regard for their antigenic specificities, leading to an overwhelming inflammatory response and shock syndrome

MRSA

S. aureus strain that contains the SCCmec plasmid, which encodes for the altered PBP2a protein

• PBP2a has low affinity for B-lactams, allowing the organism to continue functioning and dividing

HCA-MRSA: predominately bloodstream infections, pneumonia, or co-infections

CA-MRSA: soft tissue infections, sepsis, bone and joint infections

VISA/VRSA

Vancomycin resistant S. aureus strains contain the vanA gene

• MIC <2 = susceptible

• MIC 4-8 = intermediate

• MIC >16 = resistant

Staphylococcus epidermidis

• Most frequent; 50-80% of isolates

• Pathogen in endocarditis, UTIs, surgical wound infections, prosthetic devices, CSF shunts, peritoneal dialysis, occular infections

• High prevalence of methicillin resistance

• Virulence

  • Biofilms

  • Adhesion molecules

  • Polystyrene-binding proteins

• Polymyxin B ( R )

• Mannitol ( - )

• Trehelose ( - )

Staphylococcus saprophyticus

• Acute UTIs

• URT and LRT infections (young, sexually active women)

• Novobiocin ( R )

Staphylococcus capitis

• B-hemolytic

• Urease ( - )

Staphylococcus haemolyticus

• B-hemolytic

• PYR ( + )

• Urease ( - )

Staphylococcus scheliferi

• Clumping factor [coag ( + )]

Staphylococcus lugdunesis

• Yellow colonies

• Barnyard odor

• Clumping factor

Rothia mucilafinosa

• Previously Stomatococcus; normal oral flora

• Coag ( - )

• Opportunistic infections

  • Native/prosthetic valve endocarditis

  • Head and neck trauma sepsis

  • Central/peripheral access devices

  • Extreme prematurity

  • Underlying malignancies (cancer)

• “Sticky staph” colonies

Direct gram stain smears of Staphylococcus

• Gram-positive or gram-variable

• 0.5-1.5 um

• Single, pairs, short chains, clusters

Isolation of Staphylococcus

• Inoculate onto SBA

• Heavily contaminated specimens use CNA or PEA

  • G-positive selective

• MSA differentiates S. aureus

  • Can be used for nasal culture specimens

Staphylococcus colony morphology

• Rapid growth

• 1-3 mm after 24h

• Smooth, butyrous, low convex

• Yellow, yellow-orange (S. aureus)

• Off-white, grey

• Bright white (S. saphrophyticus)

• Oxidase ( - )

Micrococcus colony morphology

• Slow growth; 48h

• 1-2 mm

• Dull, matte, waxy, high convex

• Nonpigmented

• Yellow (M. luteus)

• Orange, pink-red

• Grey-white (R. mucilaginosa)

• Oxidase ( + )

Differentiation of Staphylococcus

• Glucose ( + )

• Lysostaphin ( S )

• Furazolidone ( S )

• Bacitracin ( R )

• Modified oxidase ( - )

• Coverslip ( + )

Differentiation of Micrococcus

• Glucose ( - )

• Lysostaphin ( R )

• Furazolidone ( R )

• Bacitracin ( S )

• Modified oxidase ( + )

• Coverslip ( - )

Detection of MRSA

• MRSASelect media: pink ( + )

• GeneXpert MRSA: blood culture and skin and soft tissue

  • Detects sequences in the spa gene, SCCmec, and PBP2a gene

Catalase-negative cocci

• Streptococcus spp.

• Enterococcus

What order are the Streptococci and Enterococci classified in?

Lactobacillales

Homofermentative

Medically important Streptococci ferment glucose only into lactic acid with no gas formation

Lancefield grouping system

Classification based on the cell surface carbohydrate antigens

• Cell wall polysaccharides (groups A, B, C, F, G)

• Cell wall lipoteichoic acids (group D, Enterococcus)

Lipoteichoic acids (LTAs)

Believed to play a central role in promoting initial adherence of group A strep to pharyngeal epithelial cells and host proteins

Group A B-Hemolytic Streptococcus

Streptococcus pyogenes

• Hyaluronic acid capsule: resist complement-dependent killing by phagocytes

• M proteins: capable of switching proteins, cause repeat infection

• Streptolysin O: oxygen-labile, B-hemolysis

• Streptolysin S: oxygen-stable, hemolysis not seen

• Streptococcal pyrogenic exotoxins (SPEs): scarlet fever rash, strep TSS

• C5a peptidase

• Clinical conditions

  • Pharyngitis

  • Cutaneous infections: Impetigo, cellulitis, necrotizing fasciitis

  • TSS

  • Nosocomial infection

Streptococcal pharyngitis

• 5-15% of adult pharyngitis

• 20-30% of childhood pharyngitis (5-15 yrs)

• 2-4 day incubation

• Scarlet fever: exotoxins A, B, or C

• Complications

  • Suppurative

  • Acute rheumatic fever: carditis, polyarthritis, chorea, erythema marginatum, subcutaneous nodules

  • Acute glomerulonephritis

  • TSS

• Penicillin G ( S )

Group B B-Hemolytic Streptococcus

Streptococcus agalactiae

• Tight zone of hemolysis

• Polysaccharide capsular antigens Ia, Ib, II-IX

• C5a peptidase

• B-hemolysin: lyses pulmonary alveolar epi and endothelial cells

• PBP: resist intracellular killing

• Hyaluronic acid lyase

• Carrot broth ( + )

Streptococcus agalactiae clinical spectrum

Perinatal

• Rectovaginal colonization in 10-37% of pregnant women

• Transient, intermittent, or persistent colonization

• Prophylactic antibiotics are given during pregnancy

Neonatal

• 30-70% transiently colonized by infected mothers

• Nosocomial exposure after birth

• Early-onset: acquired by ascending infection in utero, ruptured fetal membranes, or during birth

• Late-onset: acquired from birth, or postnatally from the mother; evident from 7 days to 3 months after birth

  • Bacteremia, pneumonia, meningitis, septic shock, neutropenia

Postpartum endometritis

25% of bacteremias following C-section

25-30% of asymptomatic bacteriuria during and after pregnancy

Adults with group B disease usually have underlying disease

• Diabetes, cirrhosis, stroke, neoplasia

Penicillin G ( S )

CAMP test

Method for identifying Group B Streptococcus, which produces CAMP factor

• S. agalactiae is streaked perpendicular to S. aureus on SBA

• CAMP factor enhances the hemolytic activity of S. aureus resulting in an arrowhead-shaped zone of hemolysis

Group C B-Hemolytic Streptococcus

Streptococcus dysgalactiae equisimilus

Streptococcus dysgalactiae dysgalactiae

• Normal flora of mucous membranes

• Skin and soft tissue infection

  • Complications: endocarditis, endophthalmitis, meningitis, septic arthritis

• Penicillin and cephalosporin ( S )

Group G B-Hemolytic Streptococcus

Streptococcus dysgalactiae

Streptococcus canis

• Normal flora of GI tract, vagina, and skin

• Pharyngitis, otitis media, cellulitis, septic thrombophlebitis, endocarditis, meningitis

• Penicillin and cephalosporin ( S )

Group F B-Hemolytic Streptococcus

Streptococcus milleri/angionsus group

• Pinpoint, large zone of hemolysis

• Severe suppurative infections

  • Cellulitis, deep tissue abscesses, bacteremia, osteomyelitis, endocarditis

Streptococcus pneumoniae

• a-hemolytic

• Polysaccharide capsule: able to resist opsonization, phagocytosis, and intracellular killing

• Teichoic acids, LTAs

• Pneumolysin: interacts with host cell cholesterol to lyse, inhibit bactericidal activity, stimulate cytokines, and activates classical complement

• NanA, NanB: mediates bacterial cell adherence, important in biofilm formation

• PspA: inhibits phagocytosis by blocking complement deposition

• Emerging resistance to penicillin

Pneumococcal disease

#1 cause of CA pneumonia

Predisposition

• Bronchopulmonary disease

• Compromised humoral immunity

• Smokers, asthma, COPD, cancers, viral RTIs

Complications

• Lung abscess, pericardial infection, pleural effusion, endocarditis, HUS in children

Meningitis in newborns, children, or older adults

Otitis media

Viridians Streptococcus

• a- and non-hemolytic

• Normal respiratory and genitourinary tract flora

• Mitus/Sanguinis

• Mutans

• Anginosus

• Bovis

Mitus/Sanguinis, Mutans, Salivarius groups

• 20% of subacute bacterial endocarditis

  • Prolonged bacteremia

  • Bacteremia associated with chemotherapy or stem cell transplantation

• Dental caries (S. mutans, S. sobrinus)

Anginosis group

S. anginosus

S. constellatus

S. intermedius

• Minute colonies with a matte appearance

• Normal oropharyngeal, GI, and vaginal flora

• Purulent tissue abscesses, intra-abdominal infection (40%), endocarditis, head/neck infection

• Significant in solid organ transplants

Bovis group

Group D Streptococcus; non-hemolytic

S. gallolyticus

• Found among intestinal flora of vertebrates; overlap with Enterococcus

  • Fail to grow in 6.5% NaCl broth

• Lipoteichoic acid cell wall

• Bacteremia, meningitis, endocarditis

Enterococcus spp.

• Carries the Group D strep. antigen

• Differentiation

  • Grow in 6.5% NaCl broth

  • 10C and 45C

  • pH 9.6

  • Esculin hydrolysis with 40% bile

  • PYR ( + )

• Normal GI and biliary flora

• Resistant to antibiotics that other Streptococci are susceptible to

• Agents of nosocomial infection

  • Bacteremia, UTI, skin and soft tissue infection

  • E. faecalis: 70% of infections; 5-10% VRE

  • E. faecium: 90% of VRE

  • Serious superinfections in those receiving broad-spectrum antibiotics

High level aminoglycoside resistance (HLAR)

Aminoglycoside-modifying enzyme that alters the structure resulting in decreased binding of the drug

Vancomycin resistant Enterococcus (VRE)

vanA and vanB genes encoding for the synthesis of different peptidoglycan precursors with a decreased binding affinity for glycopeptides (vancomycin, teicoplanin)

• Vancomycin-dependent (VDE): grow only in the presence of vancomycin

Aerococcus urinae

Streptococcus-like organism

• Microaerophilic

• Grow in tetrads

• a-hemolytic

Isolated in patients >50 yrs

• Diabetes, urinary tract malignancies, urolithiasis

Direct gram stain of Streptococcus

• Pairs and chains

• S. pneumoniae appear lancet-shaped

• Viridians appear elongated

Isolation of Streptococcus

• Peptone infusion base media

  • TSA, proteose, Todd-Hewitt

• 5% sheep blood added

• Streak and stab: used to ID Group A strep with SLO and SLS that grow anaerobically

Streptococcus colony morphology

Group A, C, G

• 0.5 mm

• Translucent, matte, convex

• Occasionally mucoid

Group B

• Large, more translucent than group A

• Narrow zone of hemolysis

Group D

• Large

• a- or non-hemolytic

• Gray, smooth, opaque

Group F

• Small colonies

• Large zone of hemolysis

S. pneumoniae

• Large, intense green a-hemolysis

• Heavy encapsulation, very mucoid, several mm

• Gray

• Light encapsulation, umbilicate, smaller

Differentiation of Streptococcus

• Bacitracin susceptibility: Group A

• SXT susceptibility: Group C, F, G

• CAMP test: Group B

• Hippurate: Group B

• Bile esculin: Group D and Enterococcus

• Salt tolerance: Enterococcus

• LAP: all Strep and Enterococcus

• PYR: Group A