Streptococci and Enterococci

Question 1

Streptococci (Genus) Characteristics

Answer 1

• Gram +
• cocci in chains
• catalase -
• grow on blood agar

-hemolysis patterns

•the presence of certain carbohydrate antigens in cell-wall extracts, or a polysaccharide capsule, generally separates the high from the low virulence streptococci

Question 2

Hemolysis Patterns

Answer 2

• β-hemolysis: Colonies are surrounded by a clear zone where the erythrocytes have been completely lysed.
• α-hemolysis: Colonies show hazy (incomplete hemolysis) with a green discoloration of the agar
• gamma hemolysis = none

Question 3

Pyogenic Streptococci

Answer 3

• pyogenic: involving or realting to the production of pus
• presence of a carbohydrate in the cell wall defines the pyogenic streptococci which are then classified by the antigenic specificity (A, B, C, etc.) of that antigen (Lancefield carbohydrate)
• β-hemolysis strongly suggests that the strain has one of the Lancefield group antigens, but some Lancefield positive strains may be α-hemolytic or even nonhemolytic
• Of the groups most frequently isolated from humans (A, B, C, F, and G) groups A and B are the most frequent causes of disease.
• The Lancefield group D carbohydrate is found in the genus Enterococcus which used to be classified as streptococci.

Question 4

S. pyogenes - aka Group A streptococcus (GAS) Diseases

Answer 4

• Pharyngitis, scarlet fever, impetigo, erysipelas, toxic shock syndrome, rheumatic fever, glomerulonephritis
• Strep throat!

Question 5

Group A streptococcus (GAS) - Growth

Answer 5

• Colonies are usually compact, small, and surrounded by a zone of β hemolysis that is easily seen and sharply demarcated.
• β-hemolysis is caused by hemolysins called streptolysin S and streptolysin O (oxygenlabile)

Question 6

Group A streptococcus (GAS) - Structure

Answer 6

• The GAS cell wall is built upon a peptidoglycan matrix within which lies the group carbohydrate antigen.
• A number of other molecules such as M protein, and lipoteichoic acid are imbedded in the cell wall and may extend beyond often in association with the hairlike pili.
• GAS are divided into more than 100 serotypes based on antigenic differences in the M protein.

Question 7

Group A streptococcus (GAS) - M Protein

Answer 7

•M protein is a fibrillar coiled-coil molecule with structural homology to myosin. Its carboxy terminus is rooted in the peptidoglycan of the cell wall and the amino-terminal regions extend out from the surface.

• Specificity of the more than 100 serotypes of M protein is determined by variations in the surface portions of the molecule.
• The middle part of the molecule is conserved across many M types.
• The biologic functions of M protein can be assigned to specific domains. This includes both antigenicity and the capacity to bind other molecules like fibrinogen, serum factor H and immunoglobulins.
• These actions make M protein the major factor in both the initiation of acute infection and the development of rheumatic fever

Question 8

Group A streptococcus (GAS) - Other Surface Molecules

Answer 8

• A fibronectin binding protein F and lipoteichoic acid (LTA) are both exposed on the streptococcal surface and play a role in pathogenesis.
• A hyaluronic acid capsule is present in some strains but has no proven unique role in disease.

Question 9

Group A streptococcus (GAS) - Toxins: Streptolysin O

Answer 9

•Streptolysin O

• Streptolysin O (SLO) is a pore-forming exotoxin similar to complement and staphylococcal α-toxin
• SLO is antigenic and the quantitation of antibodies against it is the basis of a serologic test called antistreptolysin O (ASO).
• Cytokines released through the superantigen mechanism.

Question 10

Group A streptococcus (GAS) - Toxins: Streptococcal superantigen toxins (StrepSAgs)

Answer 10

•Pyrogenic Exotoxins GAS produce a family of nine superantigen (SAg) toxins called streptococcal superantigen toxins or StrepSAgs produced by roughly 10% of GAS.

• Similar in structure and biological activity to the StaphSAgs of Staphylococcus aureus.
• Multiple effects including fever, rash (scarlet fever), T-cell proliferation, B-lymphocyte suppression

Question 11

Group A streptococcus (GAS) - Pharyngitis

Answer 11

• Most common bacterial cause of pharyngitis in school-age children
• Transmission is person-to-person from the large droplets produced during coughing, sneezing, or conversation.
• GAS + StrepSAg à scarlet fever which for unknown reasons is less common now that in the past.
• Unless treated, the organism will persist for 1 to 4 weeks.

Question 12

Group A streptococcus (GAS) - Impetigo

Answer 12

• Impetigo occurs when transient skin colonization with GAS is combined with minor trauma such as insect bites.
• Spread locally by scratching and to others by direct contact

Question 13

Group A streptococcus (GAS) - Wound and Puerperal Infections

Answer 13

•As with staphylococci, transmission to patients is on the hands of people, including physicians who fail to follow recommended hand washing practices.

Question 14

Group A streptococcus (GAS) - Cellulitis (erysipelas)

Answer 14

• Deep cellulitis
• Spread
• Bacteremia

Question 15

Group A streptococcus (GAS) - Streptococcal Toxic Shock Syndrome

Answer 15

• A severe invasive and toxic form of GAS soft tissue infection
• Rapid progression to death in previously healthy persons
• Multiorgan involvement with frequent spread to the blood stream and distant organs.
• As with staphylococcal TSS the findings of shock, renal impairment, and diarrhea are related to massive cytokine release stimulated by the superantigenicity of the StrepSAgs.
• Enhanced invasiveness of group A streptococci is an added feature of STSS compared to its staphylococcal counterpart.
• Systemic spread of the GAS is common as are satellite infections.

Question 16

Group A streptococcus (GAS) - Poststreptococcal Sequelae: Acute Rheumatic Fever

Answer 16

• The association between GAS and acute rheumatic fever (ARF) is based on epidemiologic studies linking it to cases of GAS pharyngitis.
• Follows weeks after GAS pharyngitis
• ARF does not follow skin or non-respiratory infection with GAS.
• Recurrences of ARF can be triggered by infection with any GAS serotype.
• Injury to the heart caused by recurrences of ARF leads to rheumatic heart disease
• Host factors - Hyper-reactors to streptococcal products (genetic)

-Anti SLO, M protein, other

•Molecular mimicry

• Type II hypersensitivity
• Anti-M protein antibody cross-reacts with host

*Connective and neural tissue

*Sarcolemmal membranes (heart)

Question 17

Group A streptococcus (GAS) - Poststreptococcal Sequelae: Poststreptococcal Glomerulonephritis

Answer 17

• Poststreptococcal glomerulonephritis may follow either respiratory or cutaneous group A streptococcal infection.
• It is caused only by certain “nephritogenic” GAS strains.
• Type III hypersensitivity – antigen-antibody complexes

Question 18

Group A streptococcus (GAS) - Acute Infections

Answer 18

• Most important adhesins are M protein, LTA, and protein F.
• In the nasopharynx (NP) all three are involved in mediating attachment to the glycoprotein fibronectin covering epithelial surfaces.
• In NP, M protein provides an essential scaffold for LTA to reach its binding site
• In skin, M protein binds directly to keratinocytes,
• Protein F is involved primarily in adherence to antigen-presenting Langerhans cells.
• SLO mediates direct injury to host cells by membrane insertion.

Question 19

Group A streptococcus (GAS) - Acute Infections M Protein Resistance to Phagocytosis

Answer 19

• GAS have the capacity to be highly invasive. M protein also plays an essential role in GAS resistance to phagocytosis.
• Related to the ability of domains of M protein to bind fibrinogen and serum factor H leading to a diminished availability of alternative pathway generated complement component C3b
• In the presence of M protein type-specific antibody, classical pathway opsonophagocytosis proceeds. The end result is immunity to subsequent infection with the same M protein serotype.

Question 20

Group A streptococcus (GAS) - Immunity

Answer 20

• Antibody directed against M protein is protective for subsequent group A streptococcal infections.
• Protection is only for subsequent infection with strains of the same M type. This is called type-specific immunity.
• This protective IgG reverses the antiphagocytic effect of M protein.
• Streptococci opsonized with type-specific antibody bind complement C3b by the classical mechanism facilitating phagocyte recognition
• Because there are over 100 M types, subsequent infections with other M types can occur.
• In ARF patients it is the hyperreaction to self in each episode that produces the chronic cardiac lesions of rheumatic heart disease.

Question 21

Group A streptococcus (GAS) - Diagnosis

Answer 21

• Blood agar plates demonstrate β-hemolysis
• Susceptibility to the antibiotic bacitracin is an indirect but simple way of separating GAS from the other pyogenic streptococci. GAS are sensitive. The others are resistant.
• Colonies are definitively identified by serologic grouping identifying the group A antigen
• Rapid Strep Test - Detection of group A antigen extracted directly from throat swabs is rapid and specific, but only 90% sensitive compared to culture. Therefore, a positive test is considered diagnostic, but negative results must be confirmed by culture.
• High titers of antistreptolysin O (ASO) are usually found in sera of patients with rheumatic fever

Question 22

Group A streptococcus (GAS) - Treatment

Answer 22

• Group A streptococci are highly susceptible to penicillin G, the antibiotic of choice. Penicillin resistance is so far unknown.
• Adequate treatment of streptococcal pharyngitis within 10 days of onset prevents rheumatic fever

Question 23

Group A streptococcus (GAS) - Prevention

Answer 23

• Penicillin prophylaxis is used to prevent recurrences of ARF during the most susceptible ages (5 to 15 years).
• Throughout life persons with a history of rheumatic fever or known heart disease receive prophylaxis while undergoing procedures known to cause transient bacteremia, such as dental extraction.
• Treatment of the acute infection may not prevent poststreptococcal glomerulonephritis.

Question 24

Streptococcus agalactiae - aka Group B streptococcus (GBS) Diseases

Answer 24

• Neonatal sepsis and meningitis
• Group B streptococci (GBS) have all the characteristics of the pyogenic streptococci with the addition of a polysaccharide capsule composed primarily of sialic acid. The typical GBS case is a newborn in the first few days of life who is not doing well.
• Fever, lethargy, poor feeding, and respiratory distress are the most common features. Localizing findings are usually lacking and the diagnosis is revealed only by isolation of GBS from blood or cerebrospinal fluid. The mortality rate is high even when appropriate antibiotics are used.

Question 25

Group B streptococcus (GBS) - Organism

Answer 25

• In addition to the Lancefield B antigen, GBS produce polysaccharide capsules of nine antigenic types (Ia, Ib, II – VIII)
• All GBS capsules contain sialic acid

Question 26

Group B streptococcus (GBS) - Epidemiology

Answer 26

• GBS are the leading cause of sepsis and meningitis in the first few days of life.
• The organism is resident in the gastrointestinal tract with secondary spread to other sites the most important of which is the vagina.
• GBS can be found in the vaginal microbiota of 10% to 30% of women. During pregnancy these organisms may gain access to the amniotic fluid or colonize the newborn as it passes through the birth canal.
• GBS produce disease in approximately 2% of pregnancy encounters.
• Risk is much higher when factors are present which decrease the infant’s innate resistance (prematurity) or increase the chances of transmission (prolonged rupture of the amniotic membranes).
• Some infants are healthy at birth but develop sepsis 1 to 3 months later.

Question 27

Group B streptococcus (GBS) - Pathogenesis

Answer 27

• GBS disease requires the right combination of organism and host factors.
• The GBS capsular sialic acid binds serum factor H which accelerates degradation of C3b
• This makes alternate pathway mediated mechanisms of opsonophagocytosis relatively ineffective.
• Classical pathway complement mediated phagocyte recognition requires specific antibody which newborns will have only if they receive it from their mother as transplacental, IgG.

Question 28

Group B streptococcus (GBS) - Immunity

Answer 28

• Antibody is protective against GBS disease but as with GAS M protein the antibody must be specific to the infecting type of GBS.
• Fortunately, there are only nine types and type III produces the majority of cases in the first week of life.
• Antibody is acquired by GBS infection with or without disease and if present specific IgG may be transmitted by the mother transplacentally to the fetus providing protection in the perinatal period. In the presence of type specific antibody, classical pathway C3b deposition, phagocyte recognition, and killing proceed normally.

Question 29

Group B streptococcus (GBS) - Manifestations

Answer 29

• The clinical findings are nonspecific and similar to those found in other serious infections in the neonatal period.
• Respiratory distress, fever, lethargy, irritability, apnea, and hypotension are common.
• Fever is sometimes absent and the patient may even be hypothermic.
• Pneumonia is common and meningitis is present in 5% to 10% of cases, but most infections have GBS circulating in the bloodstream without localizing findings.
• Even with appropriate and prompt treatment the mortality rate for early onset GBS infection approaches 20%.
• GBS infections occur in adults as peripartum chorioamnionitis, pneumonia, and a variety of skin and soft tissue infections
• GBS are not associated with rheumatic fever, glomerulonephritis, or any other immunopathologic sequelae.

Question 30

Group B streptococcus (GBS) - Treatment

Answer 30

• Although penicillin is the treatment of choice, GBS are slightly less susceptible to β-lactams than other streptococci.
• For this reason, neonatal infections are often treated with combinations of penicillin and another agent such as vancomycin or imipenem.

Question 31

Group B streptococcus (GBS) - Prevention

Answer 31

• Current strategies for prevention of neonatal GBS disease are focused on blocking contact of the infant with the organism. In colonized women attempts to eradicate the carrier state have not been successful, but intrapartum antimicrobial prophylaxis with intravenous penicillin has been shown to reduce transmission and disease in high-risk populations.
• It is now recommended by expert obstetric and perinatology groups that all newborns at risk receive such prophylaxis
• This requires screening all expectant mothers for vaginal GBS colonization in the 3rd trimester administering prophylaxis during labor to all found to be culture positive.

Question 32

Other Pyogenic Streptococci

Answer 32

• Produce various respiratory, skin, wound, soft tissue, and genital infections, which may resemble those caused by group A and B streptococci.
• Role as a cause of sore throat is not established for any of the serogroups.
• None have been associated with poststreptococcal sequelae

Question 33

Streptococcus pneumoniae species Diseases

Answer 33

• Streptococcus genus
• Pneumonia, meningitis, otitis media
• The most common form of infection with S. pneumoniae (pneumococci) is pneumonia, which begins with fever and a shaking chill followed by signs that localize the disease to the lung. These include difficulty breathing and cough with production of purulent sputum, sometimes containing blood. The pneumonia typically fills part or all of a lobe of the lung with inflammatory cells, and the bacteria may spread to the bloodstream and thus other organs. The most important of the latter is the central nervous system, where seeding with pneumococci makes S. pneumoniae a leading cause of acute purulent meningitis.
• Pneumococci are also a leading cause of otitis media.

Question 34

S. pneumoniae - Structure

Answer 34

• Pneumococci are gram-positive, oval cocci typically arranged end to end in pairs (diplococcus) giving the cells a lancet or bullet shape.
• All virulent strains have surface capsules, composed of high-molecular-weight polysaccharide polymers
• The exact makeup of the polymer is unique and distinctly antigenic for each of more than 90 serotypes.
• Choline-binding proteins extend from the surface mediating adherence to epithelial cells
• The polysaccharide capsule of S. pneumoniae is the major determinant of virulence. Unencapsulated mutants do not produce disease in humans or laboratory animals. Like the GBS capsule, pneumococcal polysaccharide interferes with effective deposition of C3b on the organism’s surface and thus phagocyte recognition and engulfment.

Question 35

S. pneumoniae - Growth

Answer 35

• On blood agar, pneumococci produce small colonies surrounded by a zone of alphahemolysis.

Question 36

S. pneumoniae - Toxins

Answer 36

• Pneumolysin is a member of the family of transmembrane pore-forming toxins which include staphylococcalα toxin and GAS streptolysin O
• Pneumolysin’s toxicity for pulmonary endothelial cells and direct effect on cilia contributes to the disruption of the endothelial barrier. This facilitates the access of pneumococci to the alveoli and eventually their spread beyond into the bloodstream.

Question 37

S. pneumoniae - Epidemiology

Answer 37

• Worldwide, S. pneumoniae is a leading cause of pneumonia, acute purulent meningitis, bacteremia, and other invasive infections.
• Frequent cause of otitis media; a virtually universal disease of childhood.
• Infections occur throughout life, but are most common in the extremes of life, the very young (60 yrs).
• Infections are derived from colonization of the nasopharynx where pneumococci can be found in 5% to 40% of healthy persons

Question 38

S. pneumoniae - Pathogenesis

Answer 38

• Aspiration of respiratory secretions containing pneumococci is the initial event leading to pneumonia. Normally, aspirated organisms are cleared rapidly by the defense mechanisms of the lower respiratory tract, including the cough and epiglottic reflexes, the mucocilliary “blanket,” and phagocytosis by alveolar macrophages. Host factors can impair the combined efficiency of these defenses.
• Choline-binding proteins mediate adherence to nasopharyngeal epithelial cells.
• Chronic pulmonary diseases, damage to bronchial epithelium from smoking or air pollution enhance susceptibility
• Respiratory dysfunction from alcoholic intoxication, narcotics, anesthesia, and trauma are also risk factors.
• When organisms reach the alveolus the involvement of pneumococcal virulence factors operates in two stages.
• The surface capsule of intact organisms acts to block phagocytosis by complement inhibition.
• When organisms begin to disintegrate the pneumolysin is released with effects on phagocyte and immune function that contribute to injury

Question 39

S. pneumoniae - Immunity

Answer 39

• Immunity to S. pneumoniae infection is provided by antibody directed against the specific pneumococcal capsular type (type specific).
• Since the number of serotypes is large, complete immunity through natural experience is not realistic. This is why pneumococcal infections occur throughout life.

Question 40

S. pneumoniae - Diagnosis

Question 41

S. pneumoniae - Treatment

Answer 41

• For decades, pneumococci were uniformly susceptible to penicillin at concentrations below 0.1 μg/mL.
• In the late 1960’s this began to change and strains with decreased susceptibility to all βlactams began to emerge.
• These strains with higher MICs were associated with treatment failures in cases of pneumonia and meningitis.
• Resistant strains are treated with fluroquinolones, clindamycin, and third generation cephalosporins sometimes with the addition of vancomycin.

Question 42

S. pneumoniae - Resistence

Answer 42

• The resistance mechanism involves alterations in the β-lactam target, the transpeptidases (penicillin binding proteins), which crosslink peptidoglycan in cell wall synthesis.
• Resistant strains have mutations in one or more of these transpeptidases which cause decreased affinity for penicillin and other β-lactams.
• Penicillinase is not produced.

Question 43

S. pneumoniae - Prevention

Answer 43

•2 capsular polysaccharide-based vaccines

1. Purified polysaccharide (PPV) - 23 most common types

• Immune response primarily B cell mediated
• Adults 65+ (6-12 mo. after PCV)

2. Protein conjugate vaccine (PCV) – 13 types •Children < 5 years, adults > 65 yrs.

• Protein stimulates T-cell mediated mechanisms
• Polysaccharide specificity maintained

Question 44

S. viridians

Answer 44

• Although their virulence is very low, viridans strains can cause disease when they are protected from host defenses.
• The prime example is subacute bacterial endocarditis. In this disease, viridans streptococci reach previously damaged heart valves as a result of transient bacteremia associated with manipulations, such as tooth extraction. Protected by fibrin and platelets, they multiply on the valve, slowly causing local and systemic disease that is fatal if untreated.
• Extracellular production of glucans, complex polysaccharide polymers, may enhance their attachment to cardiac valves.

Question 45

Enterococci genus

Answer 45

• Gram +
• diplococci or short chains
• most common species - Group D
• bile + NaCl
• The term enterococcus derives from their common presence in the intestinal tract.
• There are multiple species (E. faecalis, E. faecium, etc.) based on biochemical and cultural features.

Question 46

Enterococci Disease

Answer 46

• Enterococci are part of the intestinal microbiota.
• Although they are capable of producing disease in many settings the hospital environment is where a substantial increase has occurred in the last two decades.
• Patients with extensive abdominal surgery, indwelling devices, or undergoing procedures like peritoneal dialysis are at greatest risk.
• Most infections are acquired from the microbiota but spread between patients has been documented.
• Little is known about pathogenic mechanisms

Question 47

Enterococci Manifestations

Answer 47

•Enterococci cause opportunistic urinary tract infections, and occasionally wound and soft tissue infections, in much the same fashion as E. coli. Infections are often associated with urinary tract manipulations, malignancies, biliary tract disease, and gastrointestinal disorders.

Question 48

Enterococci Treatment

Answer 48

• The most difficult feature of the enterococci is their high and increasing levels of resistance to antimicrobial agents. Inherently relatively resistant to beta-lactams and aminoglycosides, enterococci also have particularly efficient means of acquiring plasmid and transposon resistance genes from other bacteria.
• Ampicillin remains the most consistently active agent against enterococci.
• Recently resistance to vancomycin, the antibiotic most used for ampicillin-resistant strains has emerged.
• Strains resistant to ampicillin and vancomycin may be treated with linezolid depending on susceptibility testing.