Gram positive bacteria

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

Staphylococcus aureus

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

  1. 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

  1. Fibrin/fibrinogen binding protein (clumping factor)

Promotes attachment to blood clots and traumatized tissue

  1. Surface factors -Inhibit phagocytic engulfment (capsule,Protein A)

  1. Invasins

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.

  1. Biochemical properties - enhance survival in phagocytes (carotenoids, catalase production)
  2. 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

  1. Membrane-damaging toxins - lyse eukaryotic cell membranes

(hemolysins, leukotoxin, leukocidin)

  1. Exotoxins - damage host tissues and provoke symptoms of disease (SE, TSST, ET)
  2. 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.

Exfoliating toxin

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

Staphylococcus epidermidis

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

S. epidermidis

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

S. aureus

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

Lancefield’s classification

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

Streptococcus pyogenes

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

  1. M protein, fibronectin-binding protein (Protein F) and lipoteichoic acid for adherence

  1. Hyaluronic acid capsule as an immunological disguise and to inhibit phagocytosis; M-protein also inhibits phagocytosis

  1. Invasins such as streptokinase, streptodornase (DNase B), hyaluronidase, and streptolysins

  1. 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

Host defenses

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

Viridans streptococci

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.

Streptococcus pneumoniae

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


Non Sporing






Bacillus species

Large rods occur in chains

Mostly saprophytic on soil, water and air

Central spore, nonmotile

Bacillus cereus

Food Poisoning, Two types

  1. 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

Bacillus anthracis

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

Cutaneous anthrax

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.

Gastrointestinal anthrax

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

Virulence factors

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

Clostridium species

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


Clostridium perfringens

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

Virulence factors

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.

Clostridium difficile

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.

Clostridium tetani

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

Clostridium botulinum

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

Listeria monocytogenes

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.

Virulence Factors

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.

Actinomyces israelii

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

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