Introduction to streptococci species

8/14/2019 Introduction to streptococci species

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INTRODUCTION

The streptococci are gram-positive sphericalbacteria that characteristically form pairs orchains during growth. They are widely distributedin nature. Some are members of the normalhuman flora; others are associated with importanthuman diseases attributable in part to infection bystreptococci, in part to sensitization to them.

Streptococci elaborate a variety of extracellularsubstances and enzymes.

The streptococci are a large and heterogeneousgroup of bacteria and no one system suffices toclassify them. Yet, understanding the classificationis key to understanding their medical importance.


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CLASSIFICATION

The classification of streptococci into majorcategories has been based on a series ofobservations over many years: (1) colonymorphology and hemolytic reactions on bloodagar; (2) serologic specificity of the cell wallgroup-specific substance and other cell wall orcapsular antigens; (3) biochemical reactions and

resistance to physical and chemical factors; and(4) ecologic features. Molecular genetics have alsobeen used to study the streptococci. Combinationsof the above methods have permitted theclassification of streptococci for purposes ofclinical and epidemiologic convenience, but as theknowledge evolved, new methods have been

introduced with the result that severalclassification systems have been described. Insome cases, different species names have beenused to describe the same organisms; in otherinstances, some members of the same specieshave been included in another species orclassified separately.

Streptococcus pyogenes

Most streptococci that contain the group A

antigen are S pyogenes. It is a prototypical


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human pathogen. It is used here to illustrate

general characteristics of streptococci and

specific characteristics of the species. Spyogenes is the main human pathogen

associated with local or systemic invasion and

post streptococcal immunologic disorders. S

pyogenes typically produces large (1 cm in

diameter) zones of haemolysis around colonies

greater than 0.5 mm in diameter. They are PYR-positive (hydrolysis of L-pyrrolidonyl-2-

naphthylamide) and usually are susceptible to

bacitracin.

Diseases Attributable to Local Infection with Spyogenes and Their By-Products

Streptococcal Sore ThroatThe most common infection due to -hemolytic Spyogenes is streptococcal sore throat orpharyngitis. S pyogenes adhere to thepharyngeal epithelium by means of lipoteichoicacid-covered surface pili. The glycoproteinfibronectin (MW 440,000) on epithelial cells

probably serves as lipoteichoic acid ligand. Ininfants and small children, the sore throat occursas a sub acute nasopharyngitis with a thin serousdischarge and little fever but with a tendency ofthe infection to extend to the middle ear and themastoid. The cervical lymph nodes are usuallyenlarged. The illness may persist for weeks. In


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older children and adults, the disease is moreacute and is characterized by intense

nasopharyngitis, tonsillitis, and intense rednessand edema of the mucous membranes, withpurulent exudate, enlarged, tender cervicallymph nodes, and (usually) a high fever. Twentypercent of infections are asymptomatic. A similarclinical picture can occur with infectiousmononucleosis, diphtheria, gonococcal infection,and adenovirus infection.

S pyogenes infection of the upper respiratorytract does not usually involve the lungs.Pneumonia, when it does occur, is rapidlyprogressive and severe and is most commonly asequela to viral infections, eg, influenza ormeasles, which seem to enhance susceptibility

greatly.Streptococcal Pyoderma

Local infection of superficial layers of skin,especially in children, is called impetigo. Itconsists of superficial vesicles that break downand eroded areas whose denuded surface iscovered with pus and later is encrusted. It

spreads by continuity and is highlycommunicable, especially in hot, humid climates.More widespread infection occurs in eczematousor wounded skin or in burns and may progress tocellulitis. Group A streptococcal skin infectionsare often attributable to M types 49, 57, and 59


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61 and may precede glomerulonephritis but donot often lead to rheumatic fever.

A clinically identical infection can be caused by Saureus and sometimes both S pyogenes and Saureus are present.

Invasive Group A Streptococcal Infections,Streptococcal Toxic Shock Syndrome, and ScarletFever

Fulminant, invasive S pyogenes infections with

streptococcal toxic shock syndrome arecharacterized by shock, bacteremia, respiratoryfailure, and multiorgan failure. Death occurs inabout 30% of patients. The infections tend tofollow minor trauma in otherwise healthy personswith several presentations of soft tissueinfection. These include necrotizing fasciitis,

myositis, and infections at other soft tissue sites;bacteremia occurs frequently. In some patients,particularly those infected with group Astreptococci of M types 1 or 3, the diseasepresents with focal soft tissue infectionaccompanied by fever and rapidly progressiveshock with multiorgan failure. Erythema and

desquamation may occur. The S pyogenes of theM types 1 and 3 (and types 12 and 28) that makepyrogenic exotoxin A or B are associated withthe severe infections.

Pyrogenic exotoxins AC also cause scarlet feverin association with S pyogenes pharyngitis or


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with skin or soft tissue infection. The pharyngitismay be severe. The rash appears on the trunk

after 24 hours of illness and spreads to involvethe extremities. Streptococcal toxic shocksyndrome and scarlet fever are clinicallyoverlapping diseases.

EPIDEMIOLOGY, PREVENTION AND CONTROL

humans can be asymptomatic nasopharyngeal orperineal carriers of S pyogenes, the organismshould be considered abnormal if it is detectedby culture or other means. The ultimate sourceof group A streptococci is a person harboringthese organisms. The individual may have aclinical or subclinical infection or may be acarrier distributing streptococci directly to otherpersons via droplets from the respiratory tract orskin. The nasal discharges of a person harboringS pyogenes are the most dangerous source forspread of these organisms.

Many other streptococci (viridans streptococci,enterococci, etc) are members of the normalflora of the human body. They produce diseaseonly when established in parts of the body wherethey do not normally occur (eg, heart valves). Toprevent such accidents, particularly in the courseof surgical procedures on the respiratory,gastrointestinal, and urinary tracts that result in


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temporary bacteremia, antimicrobial agents areoften administered prophylactically to persons

with known heart valve deformity and to thosewith prosthetic valves or joints.

Control procedures are directed mainly at thehuman source:

(1) Detection and early antimicrobial therapy ofrespiratory and skin infections with group Astreptococci. Prompt eradication of streptococcifrom early infections can effectively prevent thedevelopment of poststreptococcal disease. Thisrequires maintenance of adequate penicillinlevels in tissues for 10 days (eg, benzathinepenicillin G given once intramuscularly).Erythromycin is an alternative drug, althoughsome S pyogenes are resistant.

(2) Antistreptococcal chemoprophylaxis inpersons who have suffered an attack ofrheumatic fever. This involves giving oneinjection of benzathine penicillin Gintramuscularly, every 34 weeks, or daily oralpenicillin or oral sulfonamide. The first attack ofrheumatic fever infrequently causes major heartdamage; however, such persons are particularlysusceptible to reinfections with streptococci thatprecipitate relapses of rheumatic activity andgive rise to cardiac damage. Chemoprophylaxisin such individuals, especially children, must becontinued for years. Chemoprophylaxis is not


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used in glomerulonephritis because of the smallnumber of nephritogenic types of streptococci.

An exception may be family groups with a highrate of poststreptococcal nephritis.

(3) Eradication ofS pyogenes from carriers. Thisis especially important when carriers are in areassuch as obstetric delivery rooms, operatingrooms, classrooms, or nurseries. Unfortunately,it is often difficult to eradicate -hemolytic

streptococci from permanent carriers, andindividuals may occasionally have to be shiftedaway from "sensitive" areas for some time.

TREATMENT

All S pyogenesare susceptible to penicillin G, andmost are susceptible to erythromycin. Some areresistant to tetracyclines. Antimicrobial drugs

have no effect on established glomerulonephritisand rheumatic fever. In acute streptococcalinfections, however, every effort must be madeto rapidly eradicate streptococci from thepatient, eliminate the antigenic stimulus (beforeday 8), and thus prevent poststreptococcaldisease. Doses of penicillin or erythromycin that


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result in effective tissue levels for 10 daysusually accomplish this. Antimicrobial drugs are

also very useful in preventing reinfection with -hemolytic group A streptococci in rheumaticfever patients.

Streptococcus agalactiae

These are the group B streptococci. Theytypically are -hemolytic and produce zones of


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hemolysis that are only slightly larger than thecolonies (12 mm in diameter). The group B

streptococci hydrolyze sodium hippurate andgive a positive response in the so-called CAMPtest (Christie, Atkins, Munch-Peterson).

Group B streptococci are part of the normalvaginal flora in 525% of women. Group Bstreptococcal infection during the first month oflife may present as fulminant sepsis, meningitis,

or respiratory distress syndrome. Intravenousampicillin given to mothers, who carry group Bstreptococci and are in labor, preventscolonization of their infants and group Bstreptococcal disease.


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Viridans Streptococci

The viridans streptococci include S mitis, Smutans, S salivarius, S sanguis, and others.Typically they are -hemolytic, but they may benonhemolytic. Their growth is not inhibited byOptochin, and colonies are not soluble in bile(deoxycholate). The viridans streptococci are themost prevalent members of the normal flora of

the upper respiratory tract and are important forthe healthy state of the mucous membranesthere. They may reach the bloodstream as aresult of trauma and are a principal cause ofendocarditis on abnormal heart valves. Someviridans streptococci (eg, S mutans) synthesizelarge polysaccharides such as dextrans or levansfrom sucrose and contribute importantly to thegenesis of dental caries.

In the course of bacteraemia, viridansstreptococci, pneumococci, or enterococci maysettle on normal or previously deformed heartvalves, producing acute endocarditis. Rapiddestruction of the valves frequently leads to fatal

cardiac failure in days or weeks unless aprosthesis can be inserted during antimicrobialtherapy.

Sub acute endocarditis often involves abnormalvalves (congenital deformities and rheumatic oratherosclerotic lesions). Although any organismreaching the bloodstream may establish itself on


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thrombotic lesions that develop on endotheliuminjured as a result of circulatory stresses, sub

acute endocarditis is most frequently due tomembers of the normal flora of the respiratory orintestinal tract that have accidentally reachedthe blood. After dental extraction, at least 30% ofpatients have viridans streptococcalbacteraemia. These streptococci, ordinarily themost prevalent members of the upperrespiratory flora, are also the most frequentcause of sub acute bacterial endocarditis. Thegroup D streptococci (enterococci and S bovis)also are common causes of sub acuteendocarditis. About 510% of cases are due toenterococci originating in the gut or urinary tract.

The lesion is slowly progressive, and a certainamount of healing accompanies the activeinflammation; vegetations consist of fibrin,platelets, blood cells, and bacteria adherent tothe valve leaflets. The clinical course is gradual,but the disease is invariably fatal in untreatedcases. The typical clinical picture includes fever,anemia, weakness, a heart murmur, embolicphenomena, an enlarged spleen, and renal

lesions.

-Hemolytic streptococci and enterococci vary in

their susceptibility to antimicrobial agents.

Particularly in bacterial endocarditis, antibiotic

susceptibility tests are useful to determine which


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drugs may be used for optimal therapy. Amino

glycosides often enhance the rate of bactericidal

action of penicillin on streptococci, particularlyenterococci.

Streptococcus pneumonia

The pneumococci (S pneumoniae) are gram-positive diplococci, often lancet-shaped orarranged in chains, possessing a capsule ofpolysaccharide that permits typing with specificantisera. Pneumococci are readily lysed bysurface-active agents, which probably remove or


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inactivate the inhibitors of cell wall autolysins.Pneumococci are normal inhabitants of the upper

respiratory tract of 540% of humans and cancause pneumonia, sinusitis, otitis, bronchitis,bacteraemia, meningitis, and other infectiousprocesses.

Epidemiology, Prevention, & Control

Pneumococcal pneumonia accounts for about

60% of all bacterial pneumonias. In thedevelopment of illness, predisposing factors aremore important than exposure to the infectiousagent, and the healthy carrier is more importantin disseminating pneumococci than the sickpatient.

It is possible to immunize individuals with type-specific polysaccharides. Such vaccines canprobably provide 90% protection againstbacteremic pneumonia. A polysaccharide vaccinecontaining 23 types is licensed in the UnitedStates. This vaccine is appropriate for elderly,debilitated, or immunosuppressed individuals. Apneumococcal conjugate vaccine contains

capsular polysaccharides conjugated todiphtheria CRM197 protein. This seven-valentvaccine is recommended for all children aged 223 months, to help prevent ear infections, andfor selected children aged 2459 months.


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ENTEROCOCCI

The enterococci have the group D group-specificsubstance and were previously classified as

group D streptococci. Because the group D cellwall specific antigen is a teichoic acid, it is not anantigenically good marker; enterococci areusually identified by characteristics other thanimmunologic reaction with group-specificantisera. They are part of the normal entericflora. They are usually nonhemolytic, but

occasionally -hemolytic. Enterococci are PYR-positive. They grow in the presence of bile andhydrolyze esculin (bile esculin-positive). Theygrow in 6.5% NaCl. They grow well at between10 C and 45 C whereas streptococci generallygrow at a much narrower temperature range.

They are more resistant to penicillin G than the

streptococci, and rare isolates have plasmidsthat encode for -lactamase. Many isolates arevancomycin-resistant.

There are at least 12 species of enterococci.Enterococcus faecalis is the most common andcauses 8590% of enterococcal infections, whileEnterococcus faecium causes 510%. The


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enterococci are among the most frequent causesof nosocomial infections, particularly in intensive

care units, and are selected by therapy withcephalosporins and other antibiotics to whichthey are resistant. Enterococci are transmittedfrom one patient to another primarily on thehands of hospital personnel, some of whom maycarry the enterococci in their gastrointestinaltracts. Enterococci occasionally are transmittedon medical devices.

BIBLIOGRAPHY

Bisno AL, Stephens DL: Streptococcus pyogenes.

In: Mandell, Douglas, and Bennett's Principles

and Practice of Infectious Diseases, 6th ed.

Mandell GL, Bennett JE, Dolin R (editors).Churchill Livingstone, 2005.

Ruoff KL, Whiley RA, Beighton D: Streptococcus.

In: Manual of Clinical Microbiology, 8th ed.

Murray PR et al (editors). ASM Press, 2003.


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Jawetz, Melnick, & Adelberg's Medical

Microbiology, Twenty-Fourth Edition, The

McGraw-Hill Companies, 2007.

Introduction to streptococci species

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