How can we help?

You can also find more resources in our Help Center.

65 terms


Streptococcus, Enterococcus, Aerococcus
Aerococcus viridans: opportunistic, from environment, rarely causes infections in humans
Streptococcus and Enterococcus
Streptococcus and Enterococcus: Gram (+) cocci, tend to form chains in liquid media, primarily produce lactic acid from carbohydrate fermentation, catalase negative
• A few strains of Enterococcus produce small bubbles in hydrogen peroxide due to "pseudocatalase" enzymes
• Many of both Strep., and Entero. , have group specific polysaccharide antigens in the cell wall that form the basis of Lancefield grouping and subsequent classification, now referred to as just "group antigens" instead of "Lancefield group antigens"
Streptococci group Antigens
• A: most streptococci with cell wall antigen A are Streptococcus pyogenes
• B: one species: Streptococcus agalactiae
• C: Streptococcus equisimilis, the most common isolate from humans (contracted from animals)
• F + G: rarely of significance in humans
• D: two physiologically and genetically different groups possess D antigens
Group D groups
Enterococcus group (or Group D Entercoccus)
• Three species, used to be identified as Strep.
• Enterococcus faecalis (vancomycin resistant Enterococcus VRE)
• Enteorcoccus faecium (VRE)
• Enterococcus durans
Nonenterococcus group (or Group D Non-entercoccus)
• Streptococcus equinus
• Streptococcus bovis
Non-group Antigen Streptococci
• Streptococcus pneumoniae : α-hemolytic (green), previously called Diplococcus pneumoniae
• Viridans streptococci (or Streptococcus viridans): usually α-hemolytic (green)
• Streptococcus anginosus - Streptococcus milleri group: tend to form small colonies, are enhanced or require increased CO2, may be α, β, or γ hemolytic, may or may not have a group antigen, may have a butterscotch smell present, may type in group A, C, F, G or none, appear as pinpoint colonies rather than the large colonies of true A, C, or G antigen groups
Nutritionally Variant Streptococcus: may be grouped with viridans streptococci but differ in that thiol (cysteine) or B vitamin (pyridoxal) is required for growth, DNA shows they are two distinct species, new names have been proposed
Beta Hemolytic Streptococci not A, B or D
• Frequently isolated from clinical material, ID usually from cell wall group antigen
Group C: several species are animal pathogens, cause pharyngitis, pneumonia, septicemia, and meningitis
Group F: infections in human include oral infections, abscesses of the CNS, liver and appendix, neonatal sepsis
Group G: may infect animals, cause pharyngitis, ortitis media (middle ear infection), neonatal sepsis, endocarditis, and meningitis
Streptococcus pyogenes (Group A Streptococcus or GAS): Cellular Morphology
• Gram (+) cocci
• 0.5 to 1.0 micrometer in diamter
• tend to form chains in broth
• singles, pairs, and clusters also seen
• in direct smears: smaller than Staphylococci
• elongated or oval cells generally indicate streptococci, but S.pyogenes tends to be quite spherical
• presence of chains of four or more cocci provides good evidence of a streptococci
• no capsules, spores or flagella
• can use Kikes media, enriched with blood or serum
A: Growth Requirements
• Faculative with most strains growing as well or better in anaerobic conditions
• Increased CO2 is not required
• Optimum: 35 °C
• Medium enriched with blood or serum is required for good growth
• Blood agar is recommended for isolation
A: Cultural Characteristics
• Broth: granular appearance that break up on shaking
• Overnight colonies on blood agar vary from 0.5 to 1 mm
• Color varies from gray to white
• Opaque to translucent
• Buttery in consistency, while others are hard "matt" colonies that push along the agar surface
• Surrounded by zone of β-hemolytic hemolysis, size of zone depends on type and depth of blood agar, and atmospheric conditions during incubation
A: Hemolysins: Streptolysin S
• Oxygen stable, but acid labile
• When a poor quality blood agar base is used with fermentable carbohydrate, acid is produced from fermenation by streptococci, may interfer with hemolysis due to Streptolysin S
A: Hemolysins: Streptolysin O
• Oxygen labile and may be inactivated by exposure to oxygen
• blood agar base and atmosphere during incubation both affect hemolysis
A:Genus Identification:
• Gram (+) cocci, that tend to chain
• Catalase (-)
• Nitrate (-): red after addition of zinc
• Ferment a number of carbohydrates including glucose
• S. pyogenes are recognized by colonial morphology, genus tests unnecessary for typical isolates
A: Presumptive Identification of Streptococcus pyogenes: Bacitracin:
• A/B disc, most widely used test for presumptive ID
• About 99% susceptible to low [bacitracin], while most other β-hemolytic streptococci are resistant
• Not 100% accurate as 6% of other β-hemolytic streptococci are susceptible
• May be tolerated in presumptive ID from respiratory specimens, safer to treat a pt with a non-group A Streptococci, than not to treat and risk post-streptococcal disease
• Group B,C, and G are usually the ones to give false positive results
• Discs placed on heavy inoculum area of primary blood agar plates used for throat swabs
• Zones of inhibition may be clearly seen but due to high degree of error (30%) with mixed cultures, a confirmatory test should be done with a pure culture
A: Presumptive Identification of Streptococcus pyogenes: PYR:
• L-pyrrolidonyl-beta-naphthylamide (PYR) test detects presence of enzyme L-pyrrolidonyl aminopeptidase
• May be used for presumptive ID as nearly 100% are positive while other β-hemolytic streptococci with the exception of enterococci are negative
• Use a pure culture of β-hemolytic streptococci as other genera of bacteria will give a positive test
• Some CNS in clinical specimens are PYR positive
• Few labs use for S. pyogenes from respiratory specimens, used mostly for Enterococcus
• (+): pink, (-): colorless
A: Definitive Identification:
• established by detection of specific cell wall group A antigen
• -group antigens are carbohydrate residues attached to peptidoglycan of the cell wall
• Group D is different, composed of teichoic acid, attached beneath the cell wall
• For the most part antigens are specific and provide definitive ID of β-hemolytic streptococci
• Cross reactions occasionally occur and are attributes to similar carbohydrate residues
• Antigens were originally ID by Lancefield grouping, antigen is extracted from the cell wall by enzymatic or heat-acid method
• Clear extract, then reacted with specific antisera , in classical capillary precipitin reaction
• Seldom preformed today, replaced by latex and co-agglutination slide tests
Direct Detection of Group A Antigen From Throat Swabs- 1
• Early detection allows administration of appropriate antibiotics early in the infection
• May also prevent overuse of antimicrobials, when doctor's prescribe antibiotics until the diagnosis of group A Streptococci is ruled out
• Results within ten minutes
• Test kits utilize latex and co-aggultination, and enzyme immunoassays
• Throat swab is treated with an extracting reagent to prepare group A antigen for reaction with antibody contained in the test reagent
• (+) reactions are indicated by agglutination or color changes
Direct Detection of Group A Antigen From Throat Swabs-2
• sensitivity varies with reporting direct detection to recover 90% of group A, when ten or more colonies were present in culture
• some surverys report as low as 65%, most have good specificity with few swabs giving false positives
• tests are expensive and require some degree of technical expertise
• should be preformed as soon as the swab is collected
• two swabs, one for direct detection of antigen, only when the test is negative the second is sent to the lab for culture
• problems training staff in doctor's offices, most studies used trained technologists in the lab, accuracy decreases in a less than optimal environment
• CLS Gas-D test: DNA probe for Group A Strep, instrument supplied by Genprobe Company 4 hour test
• Two swabs: throat swab automatically to DNA probe, not able to be used for anything else
A:Antimicrobial Susceptibility:
• Universally susceptible to penicillin
• When allergic to penicillin use Erythromycin (rare resistant isolates), or Tetracycline (some resistant strains), susceptibility testing recommended as some strains will be resistant
• May labs do not routinely perform susceptibility tests
• Testing may be required if resistant strains, failed treatment, or periodic monitoring for resistant strains
A: Upper Respiratory Infections
• Acute pharyngitis with strep A: most common cause of sore throat "Strep Throat"
• Sudden onset of sore throat, swollen lymph glands, fever, and headache
• Complications: ear and sinus infections, rare: meningitis
• When a pyrogenic exotoxin, is produced, pt will get scarlet fever, unless protected by antibodies from a previous infection
• Rash on upper chest, spreads, "reverse" strawberry tongue, followed by red beefy appearance (thickened)
A: Cutaneous Infections
• Pustular legions, impetigo, or infections after broken skin
• When it infects the underlying dermis: erysipelas, skin has reddish patches that enlarge, thicken and swell
• Usually preceded by pharyngitis
• Occasionally disseminate and result in septicemia
A: Invasive Infections
• Serious cases increased in last decade, e.g. "flesh eating disease"
• Called: Streptococcal toxic shock, fulminant group A infection, invasive Group A Streptococcal disease
• Necrotizing fascitis
• Cellulits with pain, fever, septic shock with decreased BP, respiratory distress, necrotizing fasciitis, renal dysfunction, liver dysfunction
• Mortality rate as high as 30-50%
• Blood culture best diagnostic specimen
• Detect and report as quickly as possible
• Pen G and erythromycin usually given
A: Post-streptococcal Diseases:
• Once symptoms become evident, pt may be culture negative for S,pyogenes
• # of AB's remain present several weeks after infection
• Antistreptolysin O (ASO): most frequently tested for, and have commercial kits available, high level indicates recent infection and possible post-infection
A: Post-streptococcal Diseases: Rheumatic Fever
• May follow pharyngitis 1-5 weeks later
• Only certain strains, thought to be presence of specific M antigens, similar to myosin in heart muscle
• Strep A antibodies attach to heart muscle, and cause inflammatory reaction, and scars heart valves, may need to be replaced
A: Post-streptococcal Diseases: Acute Glomerulonephritis
• May follow pharyngitis, of cutaneous infection, antigen-antibody complexes deposited on glomerular membranes of the kidneys
• Activates complement system, that causes damage to the membrane
• Allows blood and protein to pass into urine
• UA used to diagnose
Streptococcus agalactiae (Group B Streptococcus): Cellular Morphology:
• Gram (+) cocci
• 0.5 to 1.0 micrometer in diamter
• tend to form chains in broth
• singles, pairs, and clusters also seen
• in direct smears: smaller than Staphylococci
• elongated or oval cells generally indicate streptococci
• presence of chains of four or more cocci provides good evidence of a streptococci
• no capsules, spores or flagella
B: Growth Requirements:
• Faculative with most strains growing as well or better in anaerobic conditions
• Increased CO2 is not required
• Optimum: 35 °C
• Medium enriched with blood or serum is required for good growth
• Blood agar is recommended for isolation
B: Cultural Characteristics:
• On blood agar: approximately 1mm in diameter
• Semi-transparent, gray
• Buttery or matt consistency
• Fairly small zone of β-hemolysis
• May want to differentiate, but some Strep A may resemble them
• Occasional strain of group B is non-hemolytic
B: Genus Identification:
• Catalase negative
• Nitrate negative
B: Presumptive Identification: CAMP:
• Most group B Streptococci produce CAMP, but other β-hemolytic streptococci are negative
• A few strains of group A, may give a positive CAMP reaction
• Accuracy improves when combined with bacitracin susceptibility testing
• The few strains that are bacitracin positive and CAMP positive require definitive ID
• CAMP good for small laboratory, not technically difficult and only requires a suitable strain of S. aureus
B: Presumptive Identification: Hippurate Hydrolysis
• Originally a 48 hour test, but now modified for 4 h rapid test
• Group B Strep are positive, while other β-hemolytic are generally negative
• Some group D are positive, but these can be eliminated by including an esculin hydrolysis test
B: Definitive Identification:
• Identify group B cell wall antigen, by latex or co-agglutination group B antisera
• Listeria monocytogenes may cross agglutinate with group B antisera
• Colonies on blood agar may appear similar, a gram slide will show, Listeria is a gram positive rod
B: Direct ID of Group B Antigen from Clinical Specimens
• may cause a serious general infection of the neonate, that requires immediate diagnosis and treatment
• need for rapid diagnosis, led to adaptation of tests to detect group B antigen directly from body fluids
• latex and co-agglutination reagents are available with instructions for use
• e.g. spinal fluid (suitable when available), urine (good), serum (results not reliable)
• direct detection from vaginal swabs of mother, may guide treatment
B: Antimicrobial Susceptibility
• susceptible to penicillin
• most infections respond to treatment with penicillin of ampicillin
• some strains pen tolerant, use ampicillin and gentamicin
• erythromycin and vancomycin used of patients with pen allergies
B: Clinical Significance
• opportunist, infections in immunocomprimised pts
• infections may involve eye, ear, upper respiratory tract, surgical incision
• no post-streptococcal disease
• most serious infection is neo-natal sepsis
• may be rapid, or late onset
• rapid: infected in uterus or during birth from bacteria colonizing the vagina, complications during delivery, prematurity, and heavy colonization of the mother increase the risk of infection
• often critically ill at birth, respiratory distress, septicemia, and meningitis, mortality rate is high
• late: after the first week (4 weeks), meningitis, mortality rate is lower
• may be normal fecal organisms, and in vagina
• mothers screened at 34 weeks
• may be prescribed antibiotics
• most common cause of sepsis and death in the neonate
Group D Streptococci: Cellular Morphology:
• Similar to other streptococci but some cells may be elliptical or oval
D: Growth Requirements:
• Same as for other streptococci but growth on non-enriched media better than for groups A and B
D: Cultural Characteristics
• Overnight colonies on blood agar, are 1 to 2 mm
• Gray, convex, and buttery in consistency
• Most are γ-hemolytic or have a small zone of α-hemolytic
• Only a few strains are β-hemolytic when using sheeps blood agar
• Group D will grow on some types of MacConkey agar (normally selective for gram negative)
• Colonies are small and have a deep pink color due to lactose fermentation
• MacConkey agar supports growth of group D is usually labeled "MacConkey agar without crystal violet"
• Crystal violet is selective to prevent gram positive organisms from growing
D: Genus Identification:
• Catalase negative
• Nitrate negative
• Some strains of enterococcus may give a weak reaction in the catalase test that can be attributed to "pseudocatalase" production
D: Species Identification:
• Physiological tests or identification of the group D antigen
• Latex and coagglutination tests are available
• Group D antigen contains teichoic acids and is located beneath the cell wall, causes problems with detection
• Rapid acid extraction used with latex may not detect group D antigen, while enzyme extraction usually will
• Other genera may react with group D antisera giving a false positive result
• Physiological testing is used for group D identification, and for differentiation of enterococcal and nonenterococcal strains
D: Species Identification: Bile Esculin:
• Combines the bile tolerance and the esculin hydrolysis tests
• Only streptococci that should grow in the presence of 40% bile salts found in the bile exculin media
• Group D also hydrolyse esculin while only the occasional strain of other streptococci (viridans) will give a positive reaction
• Incidence of false positives is considered low
• Several other genera are bile esculin positive, must ID as strep first, use pure culture
• Positive results can be confidently be reported as Group D
D: Species Identification: PYR:
Differentiates enterococci from nonenterococci
• Enterococci are positive
• Nonenterococci are negative
• Remember group A are also positive
• Beta-hemolytic Streptococcus that is PYR positive can only be ID as enterococci when the bile esculin test is positive
D: Species Identification: 6.5% NaCl
• Enterococci will grow but nonenterococci will not
• Problems encountered in getting an accurate result in a reasonable amount of time
• May be done as a nutrient broth with salt added, and read as growth, no growth
• Very small amount of test media and incubation in a water bath or dry incubator works best
D: Differentiation of group D as Enterococci and Nonenterococci:
• Necessary because of difference in antimicrobial susceptibility, actual species ID only required for certain isolates
• If from blood culture, species ID is desirable, as isolation of Streptococcus bovis from a blood culture indicates neoplasm of the gastrointestinal tract, if cancer not already diagnosed, pt will likely have a through gastrointestinal tract investigation
• Species ID also important with emergence of vancomycin resistant strains of enterococci, most resistant strains are Enterococcus faecium, rather than the more common isolate Enterococcus faecalis
• Species ID requires a biochemical tests, commercial systems available
D: Antimicrobial Susceptibility:
• On all clinically significant isolates
• Non-enterococci more susceptible than enterococci
• Most strains of S. bovis are susceptible to pen but combination antimicrobial therapy is occasionally for eradication
• Enterococci known to be resistant to low concentrations of ampicillin and penicillin
• Ampicillin is concentrated in urine allowing effective treatment of enterococcal UTI's
• Enterococci: Serious infections such as endocarditis, are often treated with a combo of an aminoglycoside and a penicillin
• Synergistic effect is effective
• Some strain are acquiring a high level of resistance to aminoglycosides, making combo ineffective
• Resistance caused by transferable plasmid and increased incidence of resistance is expected
D: Clinical Significance:
• Normal fecal flora, with all five species isolated from normal
• Enterococci found in infections more often
• Enterococcus faecalis is the most common isolate
• Commonly causes UTI's, wound infections, and misc infections
• Enterococcus faecium one of the most important noscomial infections due to vancomycin resistance
• E. durans not a common human isolate
• Of nonenterococci, S. bovis is the most common isolate from man, if in blood culture associated with carcinoma of gastrointestinal tract
• Little reported about pathogenicity of S. equinus in man
Vancomycin resistant Enterococcus VRE:
• Used when high level aminoglycoside resistance
• Most VRE are resistant to all licensed antimicrobials, antibiotic treatment ineffective
• Isolation of strain in hospitals, led to strict isolation procedures and even closure of hospital wards
• Most pt's are immunocomprimised, but bacteria colonize other pts and staff, endless reservoir

• May include surveillance of pt and staff for carriers (rectal swabs), and ID of resistant strains in pt's
• Solid agar plate containing vancomycin [6 mg/L], selective agents such as sodium azide may be added to the media
• Plates are usually incubated up to 72 hours, colonies are ID by usual methods as enterococci
• Most strain are ID as Enterocci faecium
• The MIC for vancomycin may be determined by micro broth dilution methods
• Vancomycin resistant strains may be referred to a reference lab for epidemiological studies which include pulse-field gel electrophoresis and testing for genetic markers, Van A, B, and C (now do at CLS)
Streptococcus pneumoniae (Pneumococci): Cultural Characteristics:
• Gram positive cocci
• About 1 um in diameter
• Cells tend to be elongated
• Typically seen in pairs but singles and chains also seen
• Appears very elongated in clinical material, if pt has been treated with antibiotics such as pen that inhibit cell wall formation, the elongation is more obvious
• Capsules are usually present in exudate and fresh cultures
• Non-flagellated and does not form spores
• Gram positive diplococci called Lancet shapes are very diagnostic for Streptococcus pneumoniae
• If diplococci see with halo in direct smear, send premlin. Report of presumptive streptococcus pnemoniae
Streptococcus pneumoniae (Pneumococci); Growth Requirements:
• Facultative with some strain requiring increased carbon dioxide for primary isolation
• All primary cultures that may yield should be incubated in 5-10% carbon dioxide
• 35 degrees
• media enriched with blood or serum is required for growth, blood agar is a suitable medium for growth
Streptococcus pneumoniae (Pneumococci): Colonial Morphology:
• on blood agar, wet glistening and transparent
• narrow zone of α-hemolysis
• most colonies are 1-2 mm in diameter, but larger mucoid colonies measuring up to 5 mm are also seen
• surface of colony may show varying degrees of flattening
• surface of colony described as life saver, checker, nail head
• gram stains from old colonies show cell debris, and bacteria may stain gram negative, subcultures may fail to grow
• the flattening of the colony is due to the autolysis of cells because of the lytic action of hydrogen peroxide that accumulates as the bacteria grow in air
• when they are incubated anaerobically, the flat surface of colonies is not observed, as hydrogen peroxide is not produced during anaerobic incubation
• chocolate agar is not good for isolation, colonies are small and autolyze rapidly, catalase in regular blood agar decomposes hydrogen peroxide, in chocolate agar, catalase is inactivated by the heating process during the prep of chocolate agar
Streptococcus pneumoniae (Pneumococci): Blood Cultures
• S, pneumoniae grows rapidly in most blood culture media, but subcultures must be done as soon as growth is detected
• If subcultures are delayed bacterial cells autolyze rapidly and growth may not be obtained on subculture plate
Streptococcus pneumoniae (Pneumococci): Genus Identification
• Seldom requires any test procedures
• Most alpha hemolytic, gram positive cocci can be assumed to be streptococcus
• Catalase and nitrate are negative but testing is not usually necessary
Streptococcus pneumoniae (Pneumococci): Species Identification
• Differentiate from S.pneumoniae from viridans
• Group D streptococci and enterococci may also be alpha hemolytic but colonial morphology of S. pneumonaie is different
Streptococcus pneumoniae (Pneumococci): Bile Solubility
• S, pneumonaie is bile soluble, while viridans and other strep and enterococci are insoluble
• Accuracy of the test depends on the type of bile salts and the age of the colonies
• Older than 24h are more likely to be insoluble in bile
• Can be performed using colonies on primary plates, gives rapid results and is economical
• Some isolates may be insoluble, it is advisable to have second test available
Streptococcus pneumoniae (Pneumococci): Optochin Susceptibility
• Susceptible to optochin (Taxos "p" disc), while other streptococci are resistant
• Easy to perform and interpret, but requires overnight incubation
• α-hemolytic strep that are susceptible can be reported as S. pneumonaie
• less than or equal to 14 mm is sensitive, and greater than 14mm is resistant
Streptococcus pneumoniae (Pneumococci): Antigen Identification
• does not have streptococcal group antigens but other antigens can be identified using latex or coaggultination reagents
• latex and coagglutination reagents available for pneumococcal antigen in body fluids
• reagent is not sensitve enough to detect antigen in urine and serum, but will work with spinal fluid
• may also be used directly with blood culture broth
• group C streptococcal antigen will cross agglutination, can lead to erroneous reporting
• S. pneumoniae has capsular antigens and may be identified by capsular swelling following exposure to homologous antisera
• Over 80 different serological types may be identified but, info is only useful for epidemiology and research
Streptococcus pneumoniae (Pneumococci):
Antimicrobial Susceptibility:
• Drug of choice is penicillin
• Over last two decades resistant strains have emerged, susceptibility testing essential
• Screening for penicillin resistant strains is done using a disc diffusion method with a 1 ug oxacillin disc
• MH agar enriched with 5% blood is inoculated with a 0.5 Mcfarland suspension
• Incubated with carbon dioxide and zones of inhibition 20mm and greater indicate susceptibility to penicillin
• Smaller zones or colonies within the zone of inhibtion indicate resistance and MIC determinations are recommended
• Resistance to penicillin is not related to beta-lactamase
Streptococcus pneumoniae (Pneumococci): Clinical Significance
• Found in the upper respiratory tract of a certain number of healthly individuals
• Most common cause of Adult lobar pneumonia
• In most cases bacteria come from an endogenous source with predisposing factors such as viral respiratory infections and alcholism
• Sudden onset, and accompanying septicemia

• Some pt's also have a severe diarrhea that indicates a bad prognosis
• Complications: Otitis media, sinusitis, conjunctivitis, and meningitis
• Common cause of death in older people, especially with predisposing factors
• Pneumococcal vaccine given extensively of the last decade, decreased the incidence of pneumonia among the elderly
• Three bacteria are responsible for the over 90% of cases of purulent meningitis (S. pneumonaie, Haemophilus influenzae, and Neisseria meningitidis)
• S. pneumonaie is the least commonly seen
• Infections are usually in children, severe, require rapid diagnosis
• Typical CSF is turbid, with a total WBC count ranging from 2000-6000*10^6/L, predominantly neutrophils
• Elongated gram positive cells are usually seen in sediment
• Direct testing of CSF for pneumococcal antigen with latex or coagglutination reagent is helpful to confirm diagnosis
Streptococcus viridans
• Most that belong to this group are α-hemolytic (viridans means green)
• Some non-hemolytic also fall into this group
• α-hemolytic strep that are not S. pneumonaie, group D streptococci or enterococci (bile eschulin, group D antigen, bile solubility, and optochin all negative)
• non-hemolytic strep, that do not have group B or D antigens (bile eschulin, group B or D antigens all negative), occasional strain of group B is non-hemolytic
Streptococcus viridans: Cellular Morphology:
• Gram positive cocci
• About 1 um in diameter
• Chains may be formed in broth
• Smears from solid media often show pleomorphic forms that may be club-shaped resembling diptheriods
• Direct smears from the upper respiratory tract often show chains of elongated cocci adhering to epithelial cells
• Do not have spores, flagella, or capsules
Streptococcus viridans: Growth Requirements:
• Facultative organisms that grow best on enriched media such as blood agar
• Optimum temperature for growth is 35 C
• Increase CO2 not usually required
Streptococcus viridans: Cultural Characteristics:
• Most seen in clinical specimens are normal flora in the upper respiratory specimens
• Overnight colonies on blood agar are about 0.5 mm in diameter or less (pinpoint)
• Surrounded by a small zone of greenish hemolysis (α-hemolytic)
• Surface of some colonies may be flat or concave, making them look like small colonies of S. pneumonaie
• Other strains of viridans are non-hemolytic
Streptococcus viridans: Identification:
• Most α-hemolytic strep, in lab are from respiratory specimens, where differentiation btw S. pneumonaie and viridans is required
• Viridans is bile insoluble, and optochin resistant, S. pneumonaie is bile soluble, and optochin susceptible
• Non-hemolytic not encountered often but may be differentiated from group D with the bile esculin test
• Viridans is bile negative, and PYR negative, while D is positive for both
• ID to species usually requires extensive physiological testing that is generally outside of the capabilities of the small to medium lab
• Complete ID of blood culture specimens, may provide useful information to clinician but definitive of most not required
• Several commercial systems are available, supplemental conventional tests may be needed
Streptococcus viridans: Antibiotic Susceptibility:
• Performed on all isolates that cause systemic infections
• Majority of isolates are susceptible to penicillin, some show moderate resistance and require high levels of antibiotics
Streptococcus viridans: Clinical Significance
• Large portion of normal flora bacteria isolated from upper respiratory tracts
• Antibiotic therapy may reduce the numbers of α streptococcis, a upper respiratory specimen lacking viridans should be questioned if it is from another source
• Major cause of subacute bacterial endocarditis
• Infections are endogenous, predisposed by damaged or scarred heart valves
• A break in oral mucosa following dental extractions or other manipulations allows the bacteria to enter the blood stream
• The bacteria lodge and grow on scarred heart valves, resulting in episodes of bacteremia and fever, may result in serious damage to heart valves
• Several blood cultures may be required to isolate bacteria from blood