Streptococci

streptococci are gram positive, nonmotile, and catalase negative. clinically important genera include streptococcus and enterococcus. they are ovoid to spherical in shape and occur as pairs or chains. most are aerotolerant anaerobes because they grow fermentatively even in the presence of oxygen. because of their complex nutritional requirements, blood enriched medium is generally used for their isolation. diseases caused by this group of organisms include acute infections of the throat and skin caused by group a streptococci (streptococcus pyogenes) female genital tract colonization, resulting in neonatal sepsis caused by group b streptococci (streptococcus agalactiae) pneumonia, otitis media, and meningitis caused by streptococcus pneumoniae and endocarditis caused by the viridans group of streptococci.
ii. classification of streptococci
streptococci can be classified by several schemes, for example, by the hemolytic properties of the organisms, and according to the presence of specific surface antigens determined by immunologic assays.
a. hemolytic properties on blood agar
?-hemolytic streptococci cause a chemical change in the hemo - globin of red cells in blood agar, resulting in the appearance of a green pigment that forms a ring around the colony ?-hemolytic streptococci cause gross lysis of red blood cells, resulting in a clear ring around the colony. ?-hemolytic is a term applied to streptococci that cause no color change or lysis of red blood cells. the traditional division of streptococci based on the ability of the bacterial colony to hemolyze erythrocytes in the blood agar medium is still considered the first step in the classification of streptococci.
b. serologic (lancefield) groupings
many species of streptococci have a polysaccharide in their cell walls known as c-substance, which is antigenic and easily extractable with dilute acid. the lancefield scheme classifies primarily ?-hemolytic streptococci into groups a through u on the basis of their c-sub stance. the clinically most important groups of ?-hemolytic streptococci are types a and b. commercial kits in which groupspecific antisera are coupled to latex beads are now widely used for
identification of ?-hemolytic streptococci.
iii. group a ?-hemolytic streptococci
s. pyogenes, the most clinically important member of this group of gram positive
cocci, is one of the most frequently encountered bacterial pathogens of humans worldwide. it can invade apparently intact skin or mucous membranes, causing some of the most rapidly progressive infections known. a low inoculum suffices for infection. some strains of s. pyogenes cause postinfectious sequelae, including rheumatic fever and acute glomerulonephritis. nasopharyngeal carriage is common
especially in colder months and particularly among children. unlike staphylococcal species, s. pyogenes does not survive well in the environment.
instead, its habitat is infected patients and also normal human carriers in whom the organism resides on skin and mucous membranes. s. pyogenes is usually spread person to person by skin contact and via the respiratory tract.
a. structure and physiology
s. pyogenes cells usually form long chains when recovered from liquid culture but may appear as individual cocci, pairs, or clusters of cells in gram stains of samples from infected tissue. structural features involved in the pathology or identification of group a streptococci include:
1. capsule: hyaluronic acid, identical to that found in human connective tissue, forms the outermost layer of the cell. this capsule is not recognized as foreign by the body and, therefore, is nonimmunogenic. the capsule is also antiphagocytic.
2. cell wall: the cell wall contains a number of clinically important components. beginning with the outer layer of the cell wall, these components include the following
a. m protein: s. pyogenes is not infectious in the absence of m protein. m proteins extend from an anchor in the cell membrane, through the cell wall and then the capsule, with the nterminal end of the protein exposed on the surface of the bacterium. m proteins are highly variable, especially the n-terminal regions, resulting in over 80 different antigenic types. thus, individuals may have many s. pyogenes infections throughout their lives as they encounter new m protein types for which they have no antibodies. m proteins are antiphagocytic and they form a coat that interferes with complement binding.
b. group a-specific c-substance: this component is composed of rhamnose and n-acetylglucosamine. all group a streptococci, , contain this antigen.
c. protein f (fibronectin-binding protein) mediates attachment to fibronectin in the pharyngeal epithelium. m proteins and lipo - teichoic acids also bind to fibronectin.
3. extracellular products: like staphylococcus aureus s. pyogenes secretes a wide range of exotoxins that often vary from one strain to another and that play roles in the pathogenesis of disease caused by these organisms .
b. epidemiology
the only known reservoir for s. pyogenes in nature is the skin and mucous membranes of the human host. respiratory dropinglets or skin contact spreads group a streptococcal infection from person to person, especially in crowded environments such as classrooms and children’s play areas.

c. pathogenesis
s. pyogenes cells, perhaps in an inhaled dropinglet, attach to the pharyngeal
mucosa via actions of protein f, lipoteichoic acid, and m protein. the bacteria may simply replicate sufficiently to maintain themselves without causing injury in which case the patient is then considered colonized. alternatively, bacteria may grow and secrete toxins, causing damage to surrounding cells, invading the mucosa, and eliciting an inflammatory response with attendant influx of white cells, fluid leakage, and pus formation. the patient then has streptococcal pharyngitis. occasionally, there is sufficient spread that the bloodstream is
significantly invaded, possibly resulting in septicemia and/or seeding of distant sites, where cellulitis (acute inflammation of subcutaneous tissue), fasciitis (inflammation of the tissue under the skin that covers a surface of underlying tissue), or myonecrosis (death of muscle cells) may develop rapidly or insidiously. however, direct inoculation of skin from another person s infection is probably more common as the pathogenesis of streptococcal skin and soft tissue infection.
d. clinical significance
s. pyogenes is a major cause of cellulitis. other more specific syndromes
include:
1. acute pharyngitis or pharyngotonsilitis: pharyngitis is the most common type of s. pyogenes infection. s. pyogenes pharyngitis (“strep throat”) is associated with severe, purulent inflammation of the posterior oropharynx and tonsillar areas
[note: if a sunburnlike rash develops on the neck, trunk, and extremities in response to the release of pyrogenic exotoxin to which the patient does not have antibodies, the syndrome is designated scarlet fever.] many strep throats are mild, and many sore throats caused by viruses are severe. hence, laboratory confirmation is important for accurate diagnosis and treatment of streptococcal
pharyngitis, particularly for the prevention of subsequent acuterheumatic fever and rheumatic heart disease.
2. impetigo: although s. aureus is recovered from most contemporary cases of impetigo, s. pyogenes is the classic cause of this syndrome. the disease begins on any exposed surface (most commonly, the legs). typically affecting children, it can cause severe and extensive lesions on the face and limbs . impetigo is treated with a topical agent such as mupirocin, or systemically with penicillin or a first-generation cephalosporin such as cephalexin, which are effective against both s. aureus and s. pyogenes.
3. erysipelas: affecting all age groups, patients with erysipelas experience a fiery red, advancing erythema, especially on the face or lower limbs .
4. puerperal sepsis: this infection is initiated during, or following soon after, the delivery of a newborn. it is caused by exogenous transmission (for example, by nasal dropinglets from an infected carrier or from contaminated instruments) or endogenously, from the mother’s vaginal flora. this is a disease of the uterine endo
metrium in which patients experience a purulent vaginal discharge and are systemically ill.
5. invasive group a streptococcal disease: common during the first half of the century, invasive group a streptococcal (gas) disease became rare until its resurgence during the past decade. patients may have a deep local invasion either without necrosis (cellulitis) or with it (necrotizing fasciitis/myositis)
[the latter disease led to the term “flesh-eating bacteria.”] invasive gas disease often spreads rapidly, even in otherwise healthy individuals, leading to bacteremia and sepsis. symptoms may include a toxic shock–like syndrome, fever, hypotension, multiorgan involvement, a sunburn like rash, or a combination of these symptoms.
6. streptococcal toxic shock syndrome: this syndrome is defined as isolation of group a ?-hemolytic streptococci from blood or another normally sterile body site in the presence of shock and multiorgan failure. the syndrome is mediated by the production of streptococcal pyrogenic exotoxins that function as superantigens
causing massive, nonspecific t-cell activation and cytokine release. patients may initially present with flulike symptoms, followed shortly by necrotizing soft tissue infection, shock, acute respiratory distress syndrome, and renal failure. treatment must be prompt and includes antistreptococcal antibiotics, usually consisting
of high-dose penicillin g plus clindamycin.
7. post-streptococcal squeal
a. acute rheumatic fever: this autoimmune disease occurs 2 to 3 weeks after the initiation of pharyngitis. it is caused by crossreactions between antigens of the heart and joint tissues, and the streptococcal antigen (especially the m protein epitopes). it is characterized by fever, rash, carditis, and arthritis. central nervous system manifestations are also common including sydenham s chorea, symptoms of which are uncontrolled movement and loss of fine motor control. rheumatic fever is preventable if the patient is treated within the first 10 days following onset of acute pharyngitis.
b. acute glomerulonephritis: this rare, postinfectious sequel occurs as soon as 1 week after impetigo or pharyngitis ensues, due to a few nephritogenic strains of group a streptococci. antigen–antibody complexes on the basement membrane
of the glomerulus initiate the disease. there is no evidence that penicillin treatment of the streptococcal skin disease or pharyngitis (to eradicate the infection) can prevent acute glomerulonephritis.



e. laboratory identification
rapid latex antigen kits for direct detection of group a streptococci in patient samples are widely used. in a positive test, the latex particles clump together, whereas in a negative test , they stay separate, giving the suspension a milky appearance. these tests have high specificity but variable sensitivity compared with culture techniques. specimens from patients with clinical signs of pharyngitis
and a negative antigen detection test should undergo routine culturing for streptococcal identification. depending on the form of the disease, specimens for laboratory analysis can be obtained from throat swabs, pus and lesion samples, sputum, blood, or spinal fluid. s. pyogenes forms characteristic small, opalescent colonies surrounded by a large zone of ? hemolysis on sheep blood agar
[note: hemolysis of the blood cells is caused by steptolysin s, which damages mammalian cells resulting in cell lysis.] this organism is highly sensitive to bacitracin, and diagnostic disks with a very low concentration of the antibiotic inhibit growth in culture. s. pyogenes is also catalase negative and optochin resistant. group a c-substance can be identified by the precipitin reaction.
serologic tests detect a patient’s antibody titer to streptolysin o (aso test) after group a streptococcal infection. anti-dnase b titers (adb test) are particularly elevated following streptococcal infections of the skin.
f. treatment
antibiotics are used for all group a streptococcal infections. s. pyogenes has not acquired resistance to penicillin g, which remains the antibiotic of choice for acute streptococcal disease. in a penicillin allergic patient, a macrolide such as clarithromycin or azithromycin is the preferred drug .penicillin g plus clindamycin are used in treating necrotizing fasciitis and in streptococcal toxic shock syndrome. clindamycin is added to penicillin to inhibit protein (i.e., toxin) synthesis so that a huge amount of toxin is not released abruptly from rapidly dying bacteria.
g. prevention
rheumatic fever is prevented by rapid eradication of the infecting organism. prolonged prophylactic antibiotic therapy is indicated after an episode of rheumatic fever, because having had one episode of this autoimmune disease in the past is a major risk factor for subsequent episodes if the patient is again infected with s. pyogenes.
iv. group b ?-hemolytic streptococci
group b streptococci, represented by the pathogen s. agalactiae, are gram-positive, catalase-negative organisms. s. agalactiae is found in the vaginocervical tract of female carriers, and the urethral mucous membranes of male carriers as well as in the gastrointestinal (gi) tract. s. agalactiae can be transmitted sexually among adults and from an infected mother to her infant at birth. group b streptococci are a leading cause of meningitis and septicemia in neonates, with a high mortality rate. they are also an occasional cause of infections in postpartumwomen (endometritis) and individuals with impaired immune systems, in,whom the organism may cause septicemia or pneumonia. samples of blood, cervical swabs, sputum, or spinal fluid can be obtained for culture on blood agar. latex agglutination tests can also demonstrate the presence of group b antigen in these samples. group b streptococci are ? hemolytic, with larger colonies and less hemolysis than group a. most isolates remain sensitive to penicillin g and ampicillin, which are still the antibiotics of choice .in life-threatening infections, an aminoglycoside can be added to the regimen. [note: pregnant carriers should be treated with ampicillin during labor if risk factors such as premature rupture of membranes or prolonged labor are present.] intrapartum prophylaxis of group b streptococcal carriers and administration of antibiotics to their newborns reduce neonatal group b strepto - coccal sepsis by as much as 90 percent.
v. streptococcus pneumoniae (pneumococcus)
s. pneumoniae are gram-positive, nonmotile, encapsulated cocci they are lancet shaped, and their tendency to occur in pairs accounts for their earlier designation as diplodocus pneumoniae. s. pneumoniae is the most common cause of community acquired pneumonia and adult bacterial meningitis and is an important cause of otitis media, sinusitis and mastoiditis. the risk of disease is highest among young children older adults, smokers, and persons with certain chronic illnesses. like other streptococci, s. pneumonia is fastidious (has complex nutritional requirements) and routinely cultured on blood agar. it releases an ? hemolysin that damages red cell membranes, causing colonies to be ? hemolytic.

s. pneumoniae is an obligate parasite of humans and can be found in the nasopharynx of many healthy individuals. this organism is extremely sensitive to environmental agents. pneumococcal infections can be either endogenous or exogenous. for example, endogenous infection involves the spread of s. pneumoniae residing in the nasopharynx of a carrier who develops impaired resistance to the organism. susceptibility to the infection may result from, for example, general debilitation such as that caused by malnutrition or alcoholism,
respiratory damage following a prior viral infection, or from a depressed immune system. patients with sickle cell disease or those who have had their spleens removed are particularly at risk for s. pneumoniae infection. infection can also be exogenous, for example, by dropinglets from the nose of a carrier. individuals such as those described above as susceptible to endogenous infection are also most likely to be infected by the exogenous route.



b. pathogenesis
the bacterial capsule of s. pneumoniae is the most important virulence factor and is the basis for the classification of serotypes of this organism. the cell-associated enzymes pneumolysin and autolysin contribute to its pathogenicity.
1. capsule: the s. pneumoniae polysaccharide capsule is both antiphagocytic and antigenic. antiphagocytic properties of the capsule protect the bacteria from polymorphonuclear leukocyte attack, facilitating growth of the bacteria prior to the appearance of anti - capsular antibodies. there are approximately 85 distinct capsular serotypes, some of which endow strains with greater virulence than others, as reflected by the fact that about 20 serotypes account for the vast majority of pneumococcal infections.
2. pili: pili enable the attachment of encapsulated pneumococci to the epithelial cells of the upper respiratory tract. not all pneumococci are piliated, but those clinical isolates that express pili are more virulent. the genes required for regulation and assembly of the pilus are not present in all pneumococcal strains, but they can be horizontally transferred between strains on a pathogenicity “islet”, which is small pathogenicity island. the chromosomal region responsible for production of the pneumococcal pilus is called the rlra islet, named for the regulatory gene (rlra) that is required for expression.
3. choline-binding protein a: choline binding protein a is a major adhesin allowing the pneumococcus to attach to carbohydrates on epithelial cells of the human nasopharynx.
4. autolysins: autolysins are enzymes that hydrolyze the components of a biological cell in which it is produced. lyt a, b and c are peptidoglycan- hydrolyzing enzymes that are present in the bacterial cell wall and are normally inactive. however, these enzymes are readily activated (for example, by surface-active agents, ?-lactam antibiotics, or stationary phase), resulting in cell lysis. autolysin is thus responsible for the release of intracellular virulence factors (pneumolysin).
5. pneumolysin: although retained within the cytosol of intact pneumococci,
pneumolysin is thought to be an important virulence factor by virtue of its ability to attack mammalian cell membranes, causing lysis once it is released by autolysin from the interior of the bacterium. pneumolysin binds to cholesterol and therefore interacts indiscriminately with all cell types. this toxin stimulates production
of proinflammatory cytokines, inhibits the activity of polymorphonuclear leukocytes and activates complement.
c. clinical significance
1. acute bacterial pneumonia: a leading cause of death, especially in older adults and those whose resistance is impaired, this disease is caused most frequently by s. pneumoniae pneumonia is frequently preceded by an upper or middle respiratory viral infection, which predisposes to s. pneumoniae infection
2. otitis media: the most common bacterial infection of children, this disease (which is characterized by earache) is most frequently caused by pneumococcus, followed by haemophilus influenza and moraxella catarrhalis .the traditional empiric treatment of pneumococcal otitis media with a ?-lactam antibiotic (with or without a penicillinase-inhibitor) has been threatened by the spread of penicillin-resistant pneumococci.
3. bacteremia/sepsis: in the absence of a focus of infection bacteremia/sepsis is commonly caused by pneumococcus, especially in individuals who are functionally or anatomically asplenic. this includes people with sickle cell disease who infarct their spleen and are functionally asplenic, although they still have a remnant of anatomical spleen.
4. meningitis: h. influenzae was formerly the leading cause of bacterial
meningitis in the united states. after a vaccine was developed against this organism, s. pneumoniae became the most common cause of adult bacterial meningitis. this disease has a high mortality rate, even when treated appropriately.
d. laboratory identification
specimens for laboratory evaluation can be obtained from a naso - pharyngeal swab, blood, pus, sputum, or spinal fluid. ?-hemolytic colonies appear when s. pneumoniae is grown on blood agar overnight under aerobic conditions at 37oc. lancet-shaped, gram positive diplococci are observed on a gram stain of the sample. growth of these bacteria is inhibited by low concentrations of the surfactant optochin, and the cells are lysed by bile acids .capsular swelling is observed when the pneumococci are treated with type-specific antisera (the quellung reaction).
e. treatment & prevention
s. pneumoniae isolates were highly sensitive to penicillin g, the initial agent of choice, until the late 1980s. since then, the incidence of penicillin resistance has been increasing worldwide. the mechanism of this resistance is an alteration of one or more of the bacterium’s penicillin-binding proteins ( rather than production of ?-lactamase. modified pbps have a much-reduced affinity for penicillin g and for some, but not all, of the other ?-lactams. most resistant strains remain sensitive to third generation cephalosporins (such as cefotaxime or ceftriaxone), and all are still sensitive to vancomycin. these antibiotics are therefore the agents of choice for invasive infections by penicillin-resistant strains of s. pneumoniae
there are two types of pneumococcal vaccine: pneumococcal polysaccharide vaccine (ppv) and pneumococcal conjugate vaccine (pcv13).
1. pneumococcal polysaccharide vaccine: introduced in the united states in 1983, ppv immunizes against 23 serotypes of s. pneumonia and is indicated for the protection of high-risk individuals older than age 2 years. this vaccine protects against the pneumococcal strains responsible for 85 to 90 percent of infections, including prominent penicillin-resistant strains.
2. pneumococcal conjugate vaccine 13: the polyvalent pcv 13, licensed in the united states in 2010, is effective in infants and toddlers (ages 6 weeks to 5 years). it is made up of 13 pneumococcal antigens conjugated to crm197, a mutant nontoxic diphtheria toxin. significant declines in the incidence of invasive pneumococcal disease occurred as a result of introduction of this and an earlier generation heptavalent conjugated vaccine (pcv 7). in addition, the vaccines prevented greater numbers of invasive pneumococcal cases through indirect effects on pneumococcal transmission (that is, herd immunity) than through its direct effect of protecting vaccinated children. young children do not elicit an immune response to oligosaccharide-only vaccines. however, if the oligosaccharide is conjugated to a protein, a protective immune response develops.
vi. enterococci
enterococci contain a c-substance that reacts with group d anti sera. therefore, in the past, they were considered group d streptococci. today, dna analysis and other properties have placed them in their own genus, enterococcus. the clinically most important species are e. faecalis and e. faecium. enterococci can be ?-, ?-, or nonhemolytic, enterococci are not very virulent, but they have become prominent as a cause of nosocomial infections as a result of their multiple antibiotic resistance.
enterococci are part of the normal fecal flora. however, they can also colonize oral mucous membranes and skin, especially in hospital settings. these organisms are highly resistant to environmental and chemical agents and can persist on fomites.. diseases
enterococci seldom cause disease in normal, healthy individuals. however, under conditions in which host resistance is lowered or the integrity of the gastrointestinal or genitourinary tract has been disrupted (for example, by instrumentation), enterococci can spread to normally sterile sites, causing urinary tract infections, bacteremia/ sepsis, endocarditis, biliary tract infection, or intra-abdominal abscesses.
laboratory identification
enterococci are distinguished from the non–group d streptococci by their ability to survive in the presence of bile, and to hydrolyze the polysaccharide esculin, producing black colonies on esculin-containing plates. unlike nonenterococcal group d streptococci, enterococci grow in 6.5 percent nacl, and yield a positive pyrazin amidase (pyr) test. e. faecalis can be distinguished from e. faecium by their fermentation patterns, which are commonly evaluated in clinical laboratories

treatment & prevention
enterococci are naturally resistant to ?-lactam antibiotics and aminoglycosides, but are sensitive to the synergistic action of a combination of these classes. in the past, the initial regimens of choice were penicillin +/– streptomycin or ampicillin +/– gentamicin however, acquired resistance determinants in many current strains this synergy. in addition, isolates frequently have natural or acquired resistances to many other antibiotic classes, including glycopeptides such as vancomycin. newer antibiotics, such as the combination of quinupristin and dalfopristin, are used to treat vancomycin-resistant infections. however, some enterococcal strains are resistant to all commercially available anti - biotics. (e. faecium is more likely to be vancomycin or multiply resistant than e. faecalis)
the rise of nosocomial infections by multiple drug –resistant enterococci is largely the result of selection due to high antibiotic usage in hospitals. judicious use of antibiotics is an important factor in controlling the emergence of these infections.
nonenterococcal group d streptococci
streptococcus bovis is the most clinically important of the nonenterococcus
group d streptococci. part of normal fecal flora, they are either ?? or non hemolytic. s. bovis occasionally causes urinary tract infections and endocarditis, the latter especially in association with colon cancer. the organism is bile and esculin positive, but is pyr-negative, and does not grow in 6.5 percent salt (unlike the enterococci). it tends to be sensitive to penicillin and other antibiotics.
viridans streptococci
the viridans group of streptococci includes many gram-positive, catalase- negative, ?? or ??hemolytic species that constitute the main facultative oral flora. the viridans streptococci include s mitis, s mutans, s salivarius, s sanguis, and others.. their growth is not inhibited by optochin, and colonies are not soluble in bile (deoxycholate). the viridans streptococci are the most prevalent members of the normal flora of the upper respiratory tract and are important for the healthy state of the mucous membranes there. they may reach the bloodstream as a result of trauma and are a principal cause of endocarditis on abnormal heart valves. some viridans streptococci (eg, s mutans) synthesize large polysaccharides such as dextrans or levans from sucrose and contribute importantly to the genesis of dental caries.