Streptococci and Enterococci Flashcards

Question

Group B streptococcus (GBS) - Organism

Answer 1

* 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

Answer 2

* 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.

Answer 3

* 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.

Answer 4

* 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.

Answer 5

* 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.

Answer 6

* 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.

Answer 7

* 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.

Answer 8

* 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

Answer 9

* 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.

Answer 10

* 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.

Answer 11

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

Answer 12

* 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.

Answer 13

* 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

Answer 14

* 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

Answer 15

* 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.

Answer 16

* 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.

Answer 17

* 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.

Answer 18

•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

Answer 19

* 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.

Answer 20

* 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.

Answer 21

* 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

Answer 22

•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.

Answer 23

* 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.