clinically important gram + bacteria II

streptococci

diverse group of organisms

may different streptococci may normally colonize mucosal membranes

• predominant component of respiratory, GI and genital tract

• many of low virulence

• however may also invade normally sterile body sites, causing significant disease

genus streptococci

gram positive cocci that may be in chains or pairs

optimal growth on media supplemented with blood

most are facultative anaerobes - will grow both aerobically and anaerobically

some are strict anaerobes - will only grow in the absence of oxygen

catalase negative - used to differentiate streptococci (also gram positive coccus)

may cause haemolysis

classification

many different streptococci - useful to classify them to aid identification of them

most commonly used classification systems:

• based on their on blood-containing agar (hemolysis)

• based on antigens contained in their cell walls (Lancefield classification)

• molecular classification (newer) - based on emm gene

• some clinically important streptococci often referred to by both Lancefield group and hemolysis pattern

Lancefield classification

serological system of grouping streptococci, based on specific antigens present in their cell walls

group A-G of most clinical significance - except for group D, all are also beta hemolytic

basis of test:

• antibody/antigen reaction

• positive if agglutination is detected

clinically important streptococci

• Strep. pyogenes (group A, beta hemolytic)

• Strep. agalactiae (group B, beta hemolytic)

• Strep. pneumoniae (alpha hemolytic)

• viridians streptococci (alpha hemolytic)

• enterococci (group D, beta or non hemolytic)

• Pepto streptococcus (anaerobic or non-hemolytic)

streptococcus pyogenes

beta hemolytic streptococci that possess the Lancefield gorup A antigen

one of the most virulent of the streptococci species. Large range of clinical presentations. Pyogenic/suppurative (resulting in pus): pharyngitis, scarlet fever, erysipelas, cellulitis, toxic shock syndrome, pneumonia

non-suppurative: post-infectious immune-mediated complications

• rheumatic fever

• glomerulonephritis

group of streptococci: epidemiology

commonly colonize the oropharynx of children and young adults

transmission via droplets

colonization is transient: influenced by acquired immunity and competition from other organisms in the oropharynx

are the major cause of bacterial pharyngitis: crowding e.g. at day-care centers and schools, facilitates spread

streptococcus pyogenes toxins and virulence factors

streptococcal pharyngitis

one of the most common bacterial infections of childhood

occasionally due to group C or group G

person-to-person spread by droplets of saliva or nasal secretions - facilitated by overcrowding

incubation period 2-4 days

streptococcal pharyngitis clinical features and diagnosis

history: patient complains of sore throat, fever, headache, nausea and vomiting

examination: redness, oedema, lymphoid hyperplasia, enlarged tonsils with exudate, enlarged tender lymph nodes, fever

lab findings: positive throat culture, immunological (antibody) response

scarlet fever

occurs when group A streptococcal strain is lysogenized by a bacteriophage (bacterial virus( that stimulates production of pyrogenic toxin

pharyngitis + rash

scarlet fever complication

supportive complications include: otitis media, acute mastoiditis

non suppurative sequelae include: acute glomerulonephritis, acute rheumatic fever

main reason for antibiotic treatment is to prevent rheumatic fever - appropriate antibiotics include oral amoxicillin or oral clarithromycin if allergic to beta lactams

rheumatic fever and glomerulonephritis

post streptococcal auto immune complications

affect a minority of people who have group A streptococcal infection

type III hypersensitivity reaction

development of antibodies to some fraction of the organism

• in rheumatic fever, the antibodies cross react with cardiac tissue → immune complex deposition on the heart

• in post streptococcal glomerulonephritis, immune complexes are deposited on the glomerular basement membrane

acute rheumatic fever

associated with streptococcal pharyngitis but not with streptococcal skin infections

fever, joint pains and carditis - may also get neurological involvement (Syndenha’s chorea)

with recovery affected heart valves become thickened and deformed - antibiotic prophylaxis required if undergoing procedure that may put patient at risk of endocarditis

diagnosis is based on the jones criteria

acute glomerulonephritis

associated with streptococcal pharyngitis and sometimes with streptococcal skin infections

the patient develops oedema, puffy face, swollen extremities (due to sodium and water retention)

hypertension, with albumin and blood in the urine

majority of young patients recover completely but may lead to permanent renal damage

erysipelas

acute onset of symptoms with systemic manifestations including fever and chills. Skin is raised and differentiated from uninvolved skin

treat with oral benzylpenicillin (5-7days). if penicillin allergy, 1st gen cephalosporin or a macrolide cold press, hydration

necrotizing fasciitis

blistering, skin discoloration due to necrosis, destruction of skin, subcutaneous and peri muscular fat with necrotic liquefaction of fatty tissue. 20-4-% mortality

once culture is confirmed, treat with high dose IV benzylpenicillin (7-10 days). surgical debridement

streptococcus agalactiae

group B beta hemolytic streptococci

colonize the lower gastrointestinal tract and the genitourinary tract

different serotypes based on capsular polysaccharides; types 1a, 111 to V are most commonly associated with colonization and disease

important cause of neonatal sepsis. Risk factors: maternal colonization, premature delivery, prolonged rupture of membranes, intra partum fever

alpha hemolytic streptococci

viridians streptococci

streptococcus pneumoniae

viridians streptococci

present in oropharynx, GIT and genitourinary tract - normal flora

invasive disease often related to breech in mucosal surfaces

several associated with:

• dental caries (strep. mutans)

• ineffective endocarditis (strep. mitis, strep. bovis)

• brain and liver abscesses (strep. anginosus)

most lack Lancefield antigens

species identified based on biochemical tests

ineffective endocarditis

vegetation on heart valve

drug choice depends on:

• causative pathogen

• presence of prostheses

• drug allergy history

• renal function

• risk of MDR organism

• social history i.e. IV drug use

combination of antibiotics usually required for 4-6 weeks

usually IV amoxicillin + gentamicin for IE caused by alpha-hemolytic streptococci

streptococcus pneumoniae

gram positive cocci in chains or diplococci

carried in the nasopharynx

polysaccharide capsule > 90 serotypes.

• allow typing strains

• vaccines available against some serotypes

pathogenesis of bacteria

portal of entry colonizes oropharynx mediated by adhesin

attach to cells

defeat/evade immune system: risk factors - immunodeficiency, splenectomy. antibiotic resistance facilitates disease

cause damage to host cells: pneumolysin and cell wall polysaccharide activates complement and cytokine release

get out and spread further: direct person to person via respiratory droplets

pneumonal infections and management

enterococci

previously classified as group D streptococci; typically seen in short chains on gram stain

facultative anaerobes

hemolytic pattern varies .. alpha or beta

grow in presence of bile salts i.e. grow on Mac Conkey agar

18 species in this genus

commensals with low virulence - bowel flora

enterococcal infection at risk patients

recent surgery

underlying disease - malignancy, burns or trauma

recent antibiotics - cephalosporins or aminoglycosides

prolonged hospitalization especially in ICU

enterococci infection clinical syndromes

urinary tract infection

endocarditis

bloodstream infection

wound infections and intra abdominal infections

important nosocomial pathogen

vancomycin resistant enterococci

problematic, particularly in hospital acquired infection

patients may become colonized ± develop significant infection due to VRE

limited treatment options - linezolid

listeria monocytogenes

gram positive bacillus, non sporing, aerobe/facultative anaerobe

transmitted by animal contact, drinking contaminated milk, eating certain soft cheeses, pate etc.

local epidemics have been associated with this

vertical transmission from mother to fetus

perinatal listeriosis

influenza like illness - usually in 3rd trimester. Fever, myalgia, backache, headaches, arthralgias

perinatal infection may result in: miscarriage or intrauterine death, premature labor and infected infant, neonatal listeriosis

can cause pneumonia, infective endocarditis, blood stream infection in immunocompromised individuals also

treatment: high dose amoxicillin plus gentamicin. resistant to cephalosporins

bacillus anthracis - causes anthrax

commonly found in soil

infection often seen in farm workers and those working with untreated leather and wool

infection depends on route of exposure i.e. wound, inhaled or gastrointestinal

anthrax toxin: 3 components - protective antigen (PA), edema factor (EF), lethal factor (LF). latter two components have enzymatic activity interfering with cAMP levels effecting water homeostasis

can be treated with antibiotics (oral doxycycline or ciprofloxacin) and antitoxin to neutralize toxin

vaccine available