• Stained positive with the Gram’s stain (dark purple)
• Contain cocci, bacilli, aerobes and anaerobes
• Produce exotoxins mainly
• Spherical bacteria (1 µm in diameter) occur in clusters resembling ‘bunch of grapes’
• Facultative anaerobes, catalase-positive and oxidase-negative
• Found in the normal flora of the nose and skin
S. aureus - large yellow (golden) colony, often hemolytic on blood agar, coagulase positive - potentially pathogenic
S. epidermidis - relatively small white colony, non hemolytic, coagulase negative --Usually nonpathogenic, could be pathogenic in hospital environment.
S. saprophyticus - coagulase negative - UTI in young women
• Non-motile, non-sporing facultative anaerobe
• Fermentation of glucose produce - lactic acid, ferments mannitol (distinguishes from S. epidermidis)
• Catalase positive
• Coagulase positive
S. aureus virulence factors
- Surface proteins
Promote colonization of host tissues; attachment to host proteins such as laminin and fibronectin - form the extracellular matrix of epithelial and endothelial surfaces
- Fibrin/fibrinogen binding protein (clumping factor)
Promotes attachment to blood clots and traumatized tissue
- Surface factors -Inhibit phagocytic engulfment (capsule,Protein A)
Promote spread in tissues - extracellular proteins (leukocidin, kinases, hyaluronidase)
• α-toxin (α-hemolysin) - most potent membrane damaging toxin for human platelets and monocytes Systemic release of α-toxin causes septic shock
• ß-toxin is a sphingomyelinase - damage cell membranes. lyse sheep erythrocytes. Human S. aureus rarely express ß-toxin.
• δ-toxin - produced by most strains of S. aureus. It is also produced by S. epidermidis. Role in disease is unknown.
• Leukocidin - a multi-component protein, damage leukocyte cell membranes, important factor in necrotizing skin infections.
- Biochemical properties - enhance survival in phagocytes (carotenoids, catalase production)
- Immunological disguises (Protein A, coagulase, clotting factor)
• Coagulase binds to prothrombin and form staphylothrombin complex - protect from phagocytic and immune defenses
• Clumping factor, the fibrinogen-binding determinant on the S. aureus cell surface
• coagulase and clumping factor are distinct entities for pathogenicity
• S. aureus express a plasminogen activator called staphylokinase - lyses fibrin
• localized fibrinolysis aids in spreading in tissues
• Proteases, lipase, deoxyribonuclease (DNAse) - provide nutrients for the bacteria
• Fatty acid modifying enzyme (FAME) important in abscesses, modify anti-bacterial lipids and prolong bacterial survival
- Membrane-damaging toxins - lyse eukaryotic cell membranes
(hemolysins, leukotoxin, leukocidin)
- Exotoxins - damage host tissues and provoke symptoms of disease (SE, TSST, ET)
- Inherent and acquired resistance to antimicrobial agents.
• Majority of clinical isolates of S aureus express a microcapsule, a surface polysaccharide of either serotype 5 or 8
• It is visualized only by electron microscopy
• Impede phagocytosis in the absence of complement
• Impede colonization of damaged heart valves, by masking adhesions
• Protein A is a surface protein of S. aureus - binds IgG disrupts opsonization and phagocytosis.
• Leukocidin specifically acts on PMNL and inhibits phagocytosis
Staphylococcal enterotoxins cause emesis (vomiting) when ingested
• Leading cause of food poisoning
• Six antigenic types (SE-A, B, C, D, E and G)
• Cause diarrhea and vomiting
• When expressed systemically, enterotoxins can also cause toxic shock syndrome.
Toxic shock syndrome toxin (TSST-1)
• Expressed systemically and cause toxic shock syndrome (TSS)
• Weakly related to enterotoxins, but no emetic activity
• Enterotoxins and TSST-1 are superantigens
• Superantigens stimulate T cells non-specifically without normal antigenic recognition
• Up to 1in 5 T cells may be activated, whereas only 1 in 10,000 is stimulated during a usual antigen presentation.
• Cytokines are released in large amounts, causing the symptoms of TSS.
• Causes scalded skin syndrome (SSS) in neonates
• Results in separation within the epidermis leaving widespread blistering
• Two antigenically distinct forms of the toxin - ETA and ETB
• Have esterase and protease activity
Virulence factors of S. aureus
Infections caused by S. aureus
• Superficial skin lesions - boils, styes and furuncle
• Serious infections - pneumonia, mastitis, phlebitis, meningitis, and UTI
• Deep-seated infections - osteomyelitis and endocarditis.
• Hospital acquired (nosocomial) infections surgical wounds, infections associated with indwelling medical devices.
• Food poisoning by releasing enterotoxins into food
• Toxic shock syndrome by release of super-antigens into the blood stream
• Scalded skin syndrome by release of exfoliation toxin
Portal of entry
• Hair follicle, usually through a break in the skin (minute needle-stick or a surgical wound
• Foreign bodies, including sutures, are readily colonized by staphylococci
• Respiratory tract
• Localized host response to staphylococcal infection is inflammation
• Elevated temperature at the site, swelling, the accumulation of pus, and necrosis of tissue
• Bacteria invasion of the blood stream - septicemia
• Bacteremia/septicemia result in internal abscesses; in lung, kidney, heart, skeletal muscle or meninges
• Produce a slime resulting in bio-film formation.
• The slime is predominantly a secreted teichoic acid, found in the cell wall of the staphylococci.
• Bio-film formation on the surface of a prosthetic devices is a significant virulence determinant
• S. epidermidis also bind to fibronectin deposited on prosthetic devices
• Phagocytosis is the major host defense against
• Bacterial capsule, protein A, Bio-film growth interfere with phagocytosis
• Antibodies are produced to neutralize toxins and promote opsonization
• Staphylococci resist destruction after phagocytic engulfment by producing carotenoids and catalase
Antimicrobial Resistance of staphylococci
• Antibiotic resistance is common in hospital strains of
• Production of ß-lactamase (penicillinase)
MRSA - methicillin resistant Staphylococcus aureus.
VRSA - vancomycin resistant Staphylococcus aureus
• It also exhibits resistance to antiseptics and disinfectants such as quaternary ammonium compounds
• If sensitive to penicillin, treated with penicillin, erythromycin, tetracyclin
• Penicillinase-resistant ß-lactams - augmentin
• Combination therapy using sulfa drugs and minocycline or rifampin
• Hospital acquired infection is often caused by antibiotic resistant strains (MRSA) - only be treated with vancomycin.
• Nonmotile, nonsporing cocci in chains or in pairs
• 0.6-1.0 µm in diameter
• Catalase-negative facultative anaerobe
Classification of streptococci
Hemolysis on blood agar
• β-hemolysis - complete lysis of red cells
• α-hemolysis - partial or "green" hemolysis
• γ-hemolysis – Non-hemolytic
Group A streptococci are nearly always β-hemolytic
Group B manifests α, β or γ hemolysis.
S. pneumoniae are α -hemolytic
Most of the oral streptococci and enterococci are non-hemolytic
Based on antigenic types
• Serologic reactivity of "cell wall" polysaccharide antigens (Rebecca Lancefield)
• The cell wall is composed of repeating units of N-acetylglucosamine (NAGA) and N-acetylmuramic acid (NAMA)
• Eighteen group-specific antigens (Lancefield groups) were established (Groups A-H and K-U)
• The Group A polysaccharide is a polymer of N-acetylglucosamine and rhamnose - also called the C substance or group carbohydrate antigen
• One of the most frequent pathogens of humans
• Belongs to Group A streptococci
• 5-15% of normal individuals harbor in respiratory tract, as normal flora
S. pyogenes virulence factors
- M protein, fibronectin-binding protein (Protein F) and lipoteichoic acid for adherence
- Hyaluronic acid capsule as an immunological disguise and to inhibit phagocytosis; M-protein also inhibits phagocytosis
- Invasins such as streptokinase, streptodornase (DNase B), hyaluronidase, and streptolysins
- Exotoxins, such as pyrogenic (erythrogenic) toxin which causes the rash of scarlet fever and systemic toxic shock syndrome.
• S. pyogenes produces multiple adhesins with varied specificities
– lipoteichoic acids (LTA)
– M protein
– fibronectin-binding proteins (Protein F)
• LTA is anchored to proteins on the bacterial surface, including the M protein
• M proteins and lipoteichoic acid are supported externally to the cell wall on fimbriae and mediate bacterial adherence to host epithelial cells
• The fibronectin-binding protein mediate adherence to the amino terminus of fibronectin on mucosal surfaces.
Extra-cellular products: invasins and exotoxins
• Streptolysin S is an oxygen-stable leukocidin
• Streptolysin O is an oxygen-labile leukocidin
• NADase is also leukotoxic
• Hyaluronidase ("spreading factor") digest host connective tissue hyaluronic acid, as well as the organism's own capsule
• Streptokinases participate in fibrin lysis
• Streptodornases A-D possess deoxyribonuclease activity; Streptodornases B and D possess ribonuclease activity as well
• Protease activity similar to that in Staphylococcus aureus has been shown in strains causing soft tissue necrosis or toxic shock syndrome
• Streptococcal pyrogenic exotoxins (SPE), also known as Erythrogenic toxin, - types A, B, C
• Act as superantigens
· Do not require processing by antigen presenting cells
· Stimulate T cells by binding class II MHC molecules directly and nonspecifically
· With superantigens about 20% of T cells may be stimulated (vs 1/10,000 T cells stimulated by conventional antigens) resulting in massive detrimental cytokine release.
• Streptococcus pyogenes - colonize and rapidly multiply and spread while evading phagocytosis and confusing the immune system.
• Acute diseases occur chiefly in the respiratory tract, bloodstream, or the skin.
• Streptococcal disease is most often a respiratory infection (pharyngitis or tonsillitis) or a skin infection (pyoderma).
• Colonization of URT and acute pharyngitis may spread to other portions of the URT or LRT resulting in otitis media, sinusitis or pneumonia
• Meningitis by direct extension of infection from the middle ear or sinuses to the meninges or by way of bloodstream
• Bacteremia lead to osteomyelitis or arthritis
• Invasive, toxigenic infections - necrotizing fasciitis, myositis and streptococcal toxic shock syndrome
• Immune-mediated post-streptococcal sequelae, - acute rheumatic fever and acute glomerulonephritis (occur in 1-3% of untreated infections)
• Puerperal fever (sepsis in the mother after childbirth)
• Scarlet fever - a severe complication of streptococcal infection - streptococcal pharyngitis accompanied by rash
• Erysipelas (a form of cellulitis accompanied by fever and systemic toxicity)
• Because of Severe invasive infections - "flesh eating bacteria"
Induction of circulating, cross-reactive antibodies
• Some of the antibodies produced during infection by certain strains of streptococci cross-react with certain host tissues
• Indirectly damage host tissues, even after the organisms have been cleared, and cause autoimmune complications
Post streptococcal sequelae
Acute rheumatic fever and acute glomerulonephritis
• Begin 1-3 weeks after an acute streptococcal illness, a latent period consistent with an immune-mediated etiology
• Whether all S. pyogenes strains are rheumatogenic is controversial; however, not all strains are nephritogenic
• Acute rheumatic fever follows pharyngeal infections only
• Acute glomerulonephritis can follow either pharyngeal or skin infections
• S. pyogenes is usually an exogenous secondary invader, following viral disease or disturbances in the normal bacterial flora
• Skin is an effective barrier against invasive streptococci, other nonspecific defense mechanisms - mucociliary movement, coughing, sneezing and epiglottal reflexes
• The host phagocytic system - second line of defense against streptococcal invasion.
• Organisms are opsonized by complement proteins and by anti-streptococcal antibodies
• S. pyogenes is rapidly killed following phagocytosis enhanced by specific antibody
• IgG antibodies against M protein promote phagocytosis - major mechanism of AMI to terminate Group A streptococcal infections
Treatment and prevention
• Penicillin is uniformly effective in treatment of Group A streptococcal disease
• Important to identify and treat Group A streptococcal infections in order to prevent sequelae
• No effective vaccine
Streptococcus agalactiae - Group B streptococci
• Members of the normal flora of female genital tract
• Cause neonatal sepsis, meningitis
• β hemolytic
Group C and G streptococci
• Occur in the nasopharynx
• Cause sinusitis, bacteremia, endocarditis
• β hemolytic
Streptococcus fecalis (enterococci) - Group D streptococci
• Part of normal enteric flora,
• Non or α hemolytic
• Common cause for nosocomial infections
• Streptococcus bovis - Non-enterococcal Group D streptococci
• Part of the enteric flora
• Occasionally cause endocarditis
• Non hemolytic
Group N streptococci
• Coagulation and souring of milk
Group EFGH and K-U streptococci
• Occur usually in animals
• α hemolytic
• Not inhibited by optochin
• Colonies not soluble in bile
• Normal flora of mouth and URT
• Principle cause for endocarditis in abnormal valves
• Cause dental caries
• Anaerobic or microaerophilic, occur in short chains, pairs or individually
• Part of the normal flora in URT, GIT, female genital tract
• Contribute to mixed anaerobic infections
• Cause brain, liver, breast, and lung abscesses.
• More than 80% of lobar pneumonia is caused by Streptococcus pneumoniae.
• Known as pneumococcus
• Gram-positive, lancet-shaped cocci (elongated cocci with a slightly pointed outer curvature).
• Usually seen as pairs of cocci (diplococci), - may also occur singly and in short chains.
• α hemolytic on blood agar, grow as glistening colonies, about 1 mm in diameter
• Fastidious bacterium, growing best in 5% carbon dioxide
• Non-sporing, and nonmotile
• Catalase negative; Ferment glucose to lactic acid.
• Streptococcus pneumoniae is a very fragile bacterium - enzymatic ability (autolysin) to disrupt and to disintegrate the cells
• Lysis usually beginning between 18-24 h after initiation of growth under optimal conditions.
• Colonies initially appear with plateau-type morphology, and then start to collapse in the centers.
• α hemolysis on blood agar
• pneumococci form a 16-mm zone of inhibition around a 5 mg optochin disc
• undergo lysis by bile salts (e.g. deoxycholate) - differentiate pneumococcus from Streptococcus viridans
• A capsule composed of polysaccharide completely envelops the pneumococcal cells
• essential determinant of virulence - interferes with phagocytosis by preventing C3b opsonization
• 90 different capsule types of pneumococci have been identified - basis of antigenic serotyping of the organism
• Anti-pneumococcal vaccines are based on formulations of various capsular (polysaccharide) antigens
• The quellung reaction (swelling reaction) is used for serotyping - the swelling of the capsule upon binding of homologous antibody
• a loopful of colony is added to equal quantity of specific antiserum - examine microscopically for capsular swelling
• Invasion with the resistance to host phagocytic response
• The cell wall components directly activate multiple inflammatory cascades - complement activation, coagulation, cytokines; IL-1, IL-6 and TNF
• When pneumococci lyse - pneumolysin and other substances are released - greater inflammation and cytotoxic effects
• Pneumolysin and hydrogen peroxide - kill cells and induce production of nitric oxide which may play a key role in septic shock
• Resistance to penicillin and cephalosporins is common
• Vaccines based on capsular polysaccharide have been formulated
Gram positive bacilli
• Large rods occur in chains
• Mostly saprophytic on soil, water and air
• Central spore, nonmotile
• Food Poisoning, Two types
- Emetic form - characterized by nausea, vomiting and abdominal cramps. Incubation period 1 - 6 h ("short-incubation" ).
• Resembles Staph. aureus food poisoning by symptoms and incubation period.
• Caused by preformed heat-stable enterotoxin
2. Diarrheal form - abdominal cramps and diarrhea with an incubation period of 8 - 16 h ("long-incubation" ). Diarrhea, small volume or profuse and watery.
• Resembles food poisoning caused by Clostridium perfringens.
• Mediated by a heat-labile enterotoxin
• Large, Gram-positive, sporing rod, 1 - 1.2µm in width x 3 - 5µm in length
• Anthrax is primarily a disease of domesticated and wild animals, particularly herbivorous animals, such as cattle, sheep, horses, mules, and goats.
• Humans become infected incidentally from diseased animals - flesh, bones, hair and excrement
• The most common form of anthrax in humans.
• Acquired via injured skin or mucous membranes - minor scratch or abrasion, usually on an exposed area of the face, neck or arms
• Characteristic gelatinous edema develops at the site. This develops into papule within 12-36 h after infection.
• The papule changes rapidly to a vesicle, then a pustule (malignant pustule), and finally into a necrotic ulcer from which infection may disseminate, giving rise to septicemia.
Inhalation anthrax (woolsorters' disease)
• Commonly from inhalation of spore-containing dust
• The disease begins abruptly with high fever and chest pain. progresses rapidly to a systemic hemorrhagic pathology and is often fatal.
• Occurs on the intestinal mucosa.
• Organisms invade the mucosa through a preexisting lesion - spread from the mucosal lesion to the lymphatic system.
Meningitis due to B. anthracis is a very rare complication
• Poly-D-glutamyl capsule -nontoxic, antiphagocytic
• Anthrax Toxin contains three components
– Edema factor (EF) - edema producing activity
– Protective antigen (PA) - induces protective antitoxic antibodies
– Lethal factor (LF) - essential for lethal effects
• Penicillin, tetracyclines and fluoroquinolones are effective against B. anthracis
• The possibility of creating aerosols containing anthrax spores has made B. anthracis a chosen weapon of bio-terrorism
• Large, pleomorphic rods that produce spores.
• found in soil, and some are normal flora of the GI tract.
• produce many enzymes - collagenase, protease, hyaluronidase, lecithinase, DNase, neuraminidase
• Soil and intestinal inhabitant.
• Forms a subterminal spore
• causes diseases such as:
– Gas gangrene. Accounts for 90% of the cases of gas gangrene.
– Soft tissue infections
– Food poisoning
– Abdominal infection of biliary tree
– Septic abortions
• α- toxin (lecithinase) - a calcium dependent phospholipase C causes lysis of RBC and other cells.
• Enterotoxin type A
– Causes acute food poisoning, which presents as a self-limited gastroenteritis.
– Heat labile.
• 25% of the diarrhea associated with antibiotic therapy.
• 95% of the cases of pseudomembranous colitis.
• C. difficile colitis is often a nosocomial (hospital-acquired) infection.
• C. difficile has a subterminal spore.
• Two toxins, A (enterotoxin) and B (cytotoxin) which are needed for enterotoxicity.
• Toxins bind to receptors on intestinal epithelial cells. -glucosylate Rho proteins and disrupt focal adhesions between cells - massive secretion of fluid (diarrhea) and an acute inflammatory infiltrate.
• C. difficile is often acquired in the hospital and colonizes the GI tract.
• Antibiotic treatment alters the balance of GI flora -overgrowth of C. difficile.
• All antibiotics can lead to C. difficile diarrhoea or colitis, -especially broad spectrum antibiotics like penicillins, cephalosporins and clindamycin.
• Diagnosed by endoscopy, and toxin assay of stool.
• Treatment: metronidazole or oral vancomycin.
• Gram-positive, spore-forming anaerobic rods
• Being punctured by a rusty nail is a common source of infection, but can also occur from a wound, a burn, an ulcer, or a compound fracture.
• Terminal spores are inoculated into site of injury. As the redox potential drops, C. tetani, an obligate anaerobe, can grow.
• Tetanus toxin is also called tetanospasmin
The Mechanism of Action of Tetanus Toxin:
• Reaches the spinal cord and brain stem via retrograde axonal transport.
• Rapidly fixed irreversibly to gangliosides at the presynaptic junctions of inhibitory motor nerve endings - blocks inhibitory impulses
• The net effect is disinhibition of motor neurons that modulate excitatory impulses resulting in increased muscle tone and painful spasms.
• This overstimulation causes spastic paralysis, instability of the autonomic nervous system, and respiratory failure.
• Lockjaw, general rigidity of the body, muscle spasms occur throughout the body
• The spores of Clostridium botulinum are oval and subterminal.
• Botulism Toxin - 7 different types.
• Botulinum neurotoxin (BoNT) is a metalloprotease.
• Binds to presynaptic sides of peripheral cholinergic synapses at ganglia and neuromuscular junctions.
• Stimulation induced acetylcholine release by presynaptic nerve terminal is irreversibly disrupted leading to flaccid paralysis.
• Return of synaptic function requires sprouting of a new presynaptic terminal with a new synapse. - requires approximately six months.
• Death occurs from respiratory failure
Floppy baby syndrome or infant botulism
Spores of Clostridium botulinum can present in honey.
When children under 1 year consume honey, they are at risk of floppy baby syndrome
Once ingested, the spores germinate and the organism colonizes the immature GI tract.
Botulism toxin produced, leads to progressive muscular weakness, and has poor motor functions.
• Wound botulism
Occurs when injured tissue is colonized by C. botulinum, with absorption of toxin from the wound site.
The infection is usually unapparent.
Wound botulism may be associated with drug abuse.
• Aerobic, nonmotile, rods
• characteristic arrangements - Chinese letters
• Corynebacterium diphtheriae, cause Diphtheria.
• rapidly developing, acute, febrile infection
• URT infection - sore throat, low-grade fever, and an adherent membrane of the tonsils, pharynx, and/or nose".
• Local lesion in the URT involves necrotic injury to epithelial cells.
• As a result, blood plasma leaks into the area and a fibrin network forms which is interlaced with rapidly-growing C. diphtheriae cells- pseudomembrane.
• Invasion of the local tissues of the throat - colonization and subsequent bacterial proliferation.
• Toxin - Diphtheria toxin kills eukaryotic cells and tissues by inhibition of protein synthesis.
• Diphtheria toxin is absorbed and disseminated through lymph and blood to the susceptible tissues of the body.
• This can lead to degenerative changes in heart, muscle, peripheral nerves, adrenals, kidneys, liver and spleen
• Acquired immunity to diphtheria -neutralizing antibody (antitoxin).
• Toxoid – attenuated toxin
• Trivalent vaccine containing diphtheria toxoid, pertussis vaccine, and tetanus toxoid (DPT or DTP vaccine).
Diphtheroids – as commensals in the throat, skin and conjunctiva
• Stain more uniformly
• Listeriosis- infection caused by eating contaminated food.
• Two main clinical manifestations - sepsis and meningitis.
• Illness is most likely to occur in pregnant women, neonates, the elderly and immunocompromised individuals.
• Listeria monocytogenes is ingested with raw, contaminated food.
• An invasin secreted by the pathogenic bacteria enables the listeriae to penetrate host cells of the epithelial lining.
• Listeria monocytogenes multiplies extracellularly and intracellularly, within macrophages after phagocytosis, or within parenchymal cells which are entered by induced phagocytosis.
• Growth at low temperatures
- grow and accumulate in contaminated food stored in the refrigerator.
- Listeriosis is usually associated with ingestion of milk, meat or vegetable products that have been held in refrigeration for a long period of time.
• Peritrichous flagella at room temperature (20-25° C), the organisms do not synthesize flagella at body temperatures (37° C).
• The bacteria move into, within and between host cells by polymerization of host cell actin at one end of the bacterium ("growing actin tails") that can propel the bacteria through cytoplasm.
Adherence and Invasion
• Attach to epithelial cells of the GI tract by means of D-galactose residues on the bacterial surface which adhere to D-galactose receptors on the host cells.
• Taken up by induced phagocytosis, analogous to the situation in Shigella.
• Gram positive branching rod
• Causes actinomycosis, chronic destructive abscesses
• The abscesses expand by burrowing through sinuses tracts to skin or internal organs with no regard for tissue planes.
• Infections are seen in the abdomen, cecum, appendix, lung, chest, face, neck, and uterus (especially with IUD use).
• Classically, “sulfur granules” are seen in pus.
• Actinomyces israelii is penicillin sensitive.
• Major member of human vaginal flora
• Member of the normal skin flora
• Contaminate samples taken through the skin – CSF, Blood
• Highly pleomorphic, Cause acne