Important human
pathogens
of Gram-positive and Gramnegative cocci
SBM 2044
Medical Microbiology
Staphylococci
Staphylococci
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Normal flora of the skin, nose, throat, and mucous
membranes; cause suppuration, abscess, pyogenic
infections, fatal septicaemia.
Haemolyse blood, coagulate plasma and produce a
variety of extracellular enzymes and toxins
At least 30 species: S aureus, S epidermidis, S
saprophyticus-UTI in young women
Staphylococci are non-motile; aerobic or microaerophilic
and relatively resistant to drying and heat
S aureus is catalase +, coagulase +; form grey to golden
yellow colonies, ferment mannitol.
Staphylococci
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Staphylococci are resistant to drugs:
1. β-lactamase production makes them resistant
to penicillins (inc ampicillin)
2. Independent β-lactamase production in
resistant to nafcillin. mecA gene in chromosome.
3. Resistant to vancomycin. Due to increased
cell wall synthesis or alterations of cell wall, or
the resistant vanA.
4. Plasmid mediated resistance to tetracyclines,
aminoglycosides.
Staphylococci - Antigenic Structure
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Antigenic peptidoglycans (polysaccharides) stimulate
production of interleukin-1 and opsonic antibodies;
chemoattractant; endotoxin-like and activate
complement.
Teichoic acids elicit anti-teichoic acid antibodies in
endocarditis patients.
Protein A binds to the Fc portion of IgG molecules,
agglutinate bacteria.
Capsules inhibit phagocytosis
Coagulase is a clumping factor on cell wall surface,
binds to fibrinogen and yield bacterial aggregation.
Enzymes and toxins
1. catalase
 2. coagulase and clumping factor – clots
oxalated or citrated plasma. Coagulase
binds to prothrombin, polymerize fibrin.
 Hyaluronidase – spreading factor;
staphylokinase – fibrinolysis; lipase.
 Toxic shock syndrome toxin – TSST-1 is
the superantigen; binds to MHC class II.
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Regulations of vir determinants
Protein expressions depend on the
bacterial growth phase.
 Accessory global regulon gene, agr has 2
operons: 1st encodes RNA III that induces
up-regulation of the secreted proteins and
down-regulation of surface proteins.
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Pathology
Furuncle or other localized abscess
 Accumulation in a hair follicle→tissue
necrosis. Coagulase coagulates fibrin,
resulting in fibrous tissue. Center of the
lesion turns to liquefaction of the necrotic
tissue→granulation tissue→healing.
 Suppuration via lymphatics or veins
 Spreading of toxins, eg. TSST-1.
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Clinical findings
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Localised staph infection appears as “pimple” hair follicle
infection, or abscess.
Dissemination of S. aureus→endocarditis, meningitis,
pulmonary infection. 2° infection: symptoms like organ
dysfunction or intense focal suppuration
Food poisoning due to staph enterotoxin presented with
short incubation (1-8 hr); violent nausea, vomiting,
diarrohea, rapid convalescence, no fever.
TSS manifested by abrupt onset of high fever, vomiting,
diarrhoea, scarlatiniform rash, cardiac/renal failures.
Women with tampons, or men and children with injured
wounds. Virtually never in bloodstream.
MRSA = multiple resistant S aureus,
because the organisms rapidly develop
resistance to many antimicrobial drugs,
drugs cannot act in the central necrotic
part of a suppurative lesion.
 Emergence resistant to erythromycin
group – not used singly for treatment of
chronic infection.
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The Streptococci
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Heterogenous group of bacteria, characterised
by colony growth characteristics, haemolysis
patterns on blood agar, antigenic composition of
group-specific cell wall.
Spherical cocci in chains, Gram +.
Most group A, B and C produce hyaluronic acid
capsules.
Cell wall contains proteins (M, T antigens),
carbohydrates and peptidoglycans, . M-protein is
hairl-like fimbriae.
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Streptococci
Streptococci
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Grow in blood or tissue fluids, 5% CO2 at 37°C. Group D
enterococci grow well at 15°C and 45°C.
Classification is based on: group-specific cell wall Ag; Mprotein; T-antigen and nucleoproteins.
-cell wall Ag by Lancefield grouping, by extraction of
centrifuged culture with hot HCl, nitrous acid or
formamide; by enzymatic lysis of strep cells (eg. Pepsin
or trypsin).chemical structure of cell wall rhamnose
acetylglucosamine.
- M-protein –virulent factor, able to resist phagocytosis
by polymorphs. There are more than 100 serotypes of M
proteins. M protein molecule rod-like coiled-coil
structure.
-T antigen – not virulent, acid-labile and heat-labile.
Toxins and Enzymes
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Streptokinase (fibrinolysin): human plasma→plasmin
(active proteolytic enzyme that digests fibrin and other
proteins). Rx of pulmonary emboli.
Hyaluronidase splits hyaluronic acid. Acts as a spreading
factor
Haemolysins: streptolysin O (SLO)-inactivated in O2,
anti-SLO level increases following infection (ASO serum
titer of 160-200 units); and streptolysin S (SLS) – zones
around colonies, elaborated in the presence of serum
Streptococcus species
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S pyogenes – Group A; β-haemolytic; human pathogen
only; diseases such as local tonsillitis, necrotizing
fasciitis and post-streptococcal glomerulonephritis.
S agalactiae- group B; β-haemolytic; NF of female
genital tract, neonatal sepsis and meningitis.
Enterococcus faecalis – normal enteric flora; γ- or αhaemolytic.
S pneumoniae – α-haemolytic; growth is inhibited by
optochin and colonies are bile-soluble.
Viridans streptococci - α-haemolytic typically; growth not
inhibited by optochin and colonies are not bile-soluble;
NF of the upper resp T; Viridans such as S mutans
synthesise large polysaccharides (dextrans) contribute to
dental caries.
Gram-negative bacilli
Pseudomonads, vibrios,
Helicobacter pylori,
pasteurella and plague
Pseudomonads
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Gram-negative bacilli; motile;
aerobic; occur widely in soil, water, plants and animals.
P aeruginosa is an obligate aerobe, sometimes
producing sweet or grape-like odour; forms smooth
round colonies with fluorescent greenish pyoverdin in
agar; bluish pigment pyocyanin.
In cystic fibrosis, P aeruginosa produces alginate, an
exopolysaccharide that forms mucoid colonies and
provide the matrix for biofilm.
Rx: combination of penicillin+aminoglycoside, not just a
single-therapy because the bacteria can rapidly develop
resistance with a single drug therapy.
P aeruginosa - toxins
Pili (fimbriae) promote attachment to epith.
 Exopolysaccharide for mucoid colonies.
 Lipopolysaccharide immunotypes;
endotoxic.
 Exotoxin A causes tissue necrosis by
inhibiting protein synthesis (like diphtheria
toxin).
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Clinical findings
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A nosocomial pathogen.
P aeruginosa infection gives rise to blue-green
pus, meningitis (lumbar puncture) and UTI.
Organism is pathogenic only when introduced
into areas devoid of normal defenses; when
intravenous or urinary catheters are used; when
neutropenia is present as in cancer chemo.
In most cases, symptoms and signs are
nonspecific and are related to organ involved.
Ecthyma gangrenosum is haemorrhagic
necrosis of the skin, where lesions are
surrounded by erythema with no pus.
Burkholderia pseudomallei
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Small, motile, aerobic G-, grows in 42°C.
Grows on standard media, colonies vary from
mucoid and smooth to rough and wrinkled; from
cream to orange in colour.
Causes melioidosis of humans, in Southeast
Asia and N Australia.
-Incubation period is short (2-3 days); localised
infection may result in acute septicaemic and
may involve many organs. Commonly
pulmonary infection (1° pneumonia)
Vibrio cholerae
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Vibrios are comma-shaped aerobic rods, motile
and possess a polar flagellum; oxidase +.
V cholerae produces enterotoxin that causes
cholera (serogroups O1 and O139).
The culture presented as convex, smooth,
opaque round colonies and granular; grow well
at 37°C on thiosulfate-citrate-bile-sucrose agar
with yellow colonies against the dark-green
b/ground. Colonies are rapidly killed by acid.
Cholera
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The connection between cholera and faecal
bacteria in drinking water was discovered in
London by John Snow in 1854.
Not an invasive infection. Organisms remain in
intestinal tract. V cholerae attach to the microvilli
of epith cells whereby they multiply and liberate
cholera toxin.
Incubation period 1-4 days, presented with
sudden onset of nausea, vomiting and profuse
diarrhoea with abdominal cramps. Stools (“rice
water”) contain mucus, epith cells and a lot of
vibrios.
Rx: water and electrolyte replacement
Helicobacter pylori
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A spiral-shaped G-; multiple flagella at one pole
and highly motile; grow at 37°C pH 6-7 and
killed in acidic; oxidase + and catalase +.
Cause gastritis, duodenal ulcer/peptic disease
and gastric carcinoma. Urease producer.
Identification: H pylori grow in 3-6 days when
incubated at 37°C in a microaerophilic
environment. Colonies are translucent; 1-2mm in
diameter.
H pylori - pathogenesis
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H pylori ingested through food, retain in gastric mucus
(physiological pH).
Produces: protease – makes mucous impermeable to
acid; urease which yields ammonia production and
buffers the acidic pH.
Presence of H pylori correlates with duodenal ulcer
disease.
Lab tests: gastric biopsies, blood for serum Abs.
Immunity: Infected patients will develop IgM Abs. later,
IgG and IgA.
Rx: Antibiotics with metronidazole, acid-suppressing
agent for healing ulcers, proton pump inhibitors to inhibit
organism (as well as urease inhibitors).
Plague
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Plague – infection of wild
rodents, transmitted
interspecies, occasionally
from rodents to humans,
by bites of fleas. Outbreak
seen in India, Africa, North/South of America and
S.E.Asia.
Black death – pandemic that killed 2/3 of Europeans
(~75mill people) in late 1340s.
Caused by Yersinia pestis, G- rod; exhibits striking
bipolar staining with Wright-Giemsa stain; non-motile.
Antigenic structure: all possess LPS, which gives
endotoxin activity when released.
Pathogenesis
1
Polymorphs→
killed
Monocytes→
survive
By coagulase
Clinical findings
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After 2-7 days of incubation, patients presented with high
fever, painful lymphadenopathy, enlarged, tender nodes
(“buboes”: hence bubonic plague) in groin and axillae.
Sometimes with vomiting and diarrhoea.
Later disseminated intravascular coagulation leads to
hypotension, altered mental status, renal & cardiac
failures.
Diagnostic: important to recognise early, confirms with
lab.
Specimens taken from blood, aspirates of enlarged
lymph nodes and sputum.
Control: (1) streptomycin, alternatively tetracycline; (2)
destruction of plague-infected rodents, (3) vaccine
(formalin-killed) for travellers.
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Swollen nodes
Pasteurella
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Gram - ; Non-motile; coccobacilli, aerobes or
facultative anaerobes.
Pasteurella multocida – worldwide in Resp T and
GIT; most common organisms in human wounds
infected by bites from cats and dogs.
History of animal bite followed within hours of
acute onset of redness, swelling and pain.
Rx: antibiotics penicillin G.
The Neisseriae
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Gram – cocci; usually in pairs, sometimes piliated. 2
major species: gonococci and meningococci –
differentiated by the usual clinical presentations of the
diseases they cause.
Cultures: after 48h on enriched media (Mueller-Hinton)
form convex, glistering, elevated, mucoid colonies, 15mm; non-haemolytic, transparent colonies.
Produce oxidase. Test: paper soaked with tetramethylparaphenylenediamine hydrochloride → dark purple
rapidly.
Rapidly killed by drying, sunlight, moist, heat and
many disinfectants.
Neisseria gonorrhoeae
Gonorrhoea "flow of seed"; in ancient times it
was thought that the pus discharge
associated with the disease contained
semen.
 Antigenic switching mechanism:
pilin ↔Opa ↔ lipooligosaccharide (LOS)
Rapid and occurs 1 in every 103 organisms
Different mechanisms applied - Multiple genes
encode for pilin,removal part of DNA for Opa.
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Neisseria gonorrhoeae
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Clinical specimens are looked for Opa protein;
colonies form transparent or opaque.
Pathogenesis: gonococci attack mucous
membranes of GUT, eyes, rectum:
Acute suppuration→tissue invasion→ chronic
inflammation→fibrosis; spread to other tissues.
Gonococcal bacteriaemia leads to skin lesions,
haemorrhagic papules and pustules.
Newborn – gonococcal ophthalmia neonatum.
Neisseria gonorrhoeae
Rx: localised infection – serum-sensitive
(killed by Abs and c’) bacteria; whereas
bacteriaemia – serum-resistant.
 Diagnostic lab tests: taken from pus &
secretions from urethra, cervix, throat,
synovial fluid and smears from endocervix
and urethral are stained.
 Disease: worldwide, sexually, multiple
sexual partners, asymptomatic infection.
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Meningococcus
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Piliated cocci; meningococci pathogenic to
human only.
Symptoms with upper Resp T, high fever,
sudden intense headache, vomiting, stiff neck,
coma within hours. Meningococcemia –
thrombosis of many small blood vessels in
multiple organs.
Specimens: blood, spinal fluid, nasopharyngeal
swab; all are tested with Gram staining and
oxidase test. Further test: Abs to meningococcal
polysaccharides by latex agglutination or
haemagglutination.
Gram-positive
F anaer
Gramnegative
Aerobes
Cocci
Bacilli
Anaerobes
F anaer
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Bacillus-S
Clostridium -S
Escherichia
Klebsiella
Salmonella
Shigella
Yersinia
Vibrio
Haemophilus
Pasteurella
Aerobes
Anaerobes
Bordatella
Neisseria
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Pseudomonas
Bacteriodes
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Pustules Pustules are well circumscribed elevations of the
superficial layers of the epidermis. Like papules, they may be
follicular or interfollicular in distribution. The most common cause is
bacterial infection, where the pustules are filled with neutrophils,
bacteria, debris, and possibly a few loose keratinocytes (acantholytic
cells). It must be kept in mind that pustules, due to the thinness of
the epidermis are usually transient. What is often observed is a
transition from erythematous macules to papules to relatively few
pustules to small crusts and scale.
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Papules P Papules are circumscribed, solid elevations of the skin,
up to 1 cm in diameter. The elevation may be due to accumulation of
cells, fluid, debris or metabolic deposits and may be follicular or
interfollicular in orientation. Papules often begin as erythematous
macules.