Medically Important Bacteria – Gram
Positive Bacilli
Learning Objectives
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Classification of GPBs
General properties of GPBs
Discuss the role of toxins and other virulence factors in
pathogenesis
Describe the mechanism of action of various toxins
List pathogenicity
Medically Important Gram-Positive Bacilli
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Can be subdivided into two general groups,
based on the presence or absence of spores
1.
2.
Spore-formers
Nonspore-formers
Scheme for Differentiating GPBs
Spore Forming
Gram Positive Bacilli
Spore Forming GPBs
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Spore forming gram +ve bacilli: 2 groups
1. Aerobic – Bacillus: Important species are
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Bacillus anthracis – Anthrax
Bacillus cereus – food poisoning
Bacillus stearothermophilus – efficiency testing of an autoclave
2. Anaerobic – Clostridia: Imp species include
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Clostridium perfringens – gas gangrene
Clostridium tetani – tetanus
Clostridium botulinum – botulism (food poisoning)
Clostridium difficile – pseudomembranous colitis
Bacillus
General Characteristics of the Genus
Bacillus
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Aerobic
Gram-positive, spore-forming
(non bulging spores), bacilli
2 species of medical
importance:
– Bacillus anthracis
– Bacillus cereus
B. anthracis - Virulence factors
Two virulence factors –
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Capsular polysaccharide – inhibits
phagocytosis
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Anthrax toxin : made up of 3 fractions
1.
2.
3.
Edema factor (EF or Factor I)
Protective antigen factor (PA or Factor II)
Lethal factor (LF or Factor III)
* They are not toxic individually, the whole
complex produces local edema &
generalised shock. Toxin production is
plasmid mediated
Anthrax toxin
Pathogenicity
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Anthrax – zoonotic disease of primarily cattle &
sheep.
Animals get infected by ingestion of spores
present in the soil
Bacilli are shed in discharges from the mouth,
nose & rectum - sporulate in the soil.
Human anthrax – contracted from animals,
directly or indirectly.
Human Anthrax
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The disease may present as
1.
2.
3.
Cutaneous anthrax : ‘Hides Porter’s disease’ – inoculation of spores
into the skin
Pulmonary anthrax : ‘Wool Sorter’s disease’ – inhalation of spores
Intestinal anthrax - ingestion of inadequately cooked meat containing
anthrax spores
* All types lead to fatal septicemia
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Human anthrax can be
1.
2.
Industrial – in meat packing or wool factories
Nonindustrial – frequent association with animals like butchers,
veterinarians, farmers
Anthrax
Bacillus cereus
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Readily isolated from soil, vegetables and a wide variety
of foods including milk, cereals, spices, poultry & meat.
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Spores survive cooking and reheating
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Causes foodborne gastroenteritis – 2 patterns of
disease (diarrhoeal & emetic); both types are mild & self
limited, requiring no specific therapy.
Bacillus cereus
clinical presentation
Gastroenteritis
DIARRHOEAL FORM
EMETIC FORM
Wide range of foods including cooked meat & vegetables
Incubation period > 6 hours
Diarrhoea & abdominal pain
Lasts 20-36 hours
Chinese fried rice exclusively
Incubation period < 6 hours
Severe vomiting
Lasts 8-10 hours
Clostridium
General Characteristics of the Genus
Clostridium
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Gram-positive, spore-forming
rods
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Anaerobic and catalase negative
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Oval or spherical (bulging) spores
produced only under anaerobic
conditions
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Cause wound infections, tissue
infections, and food intoxications
Cl. perfringens
Cl. perfringens: Virulence factors
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Classified into Five strains, A to E - based on the toxins they
produce
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Produces many toxins: 4 major toxins – alpha, beta, epsilon &
iota
1.
Alpha toxin – lecithinase C, a phospholipase
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2.
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Damages cell membranes
Increase capillary permeability
Extravasation & increased tension in affected muscles
Lysis of erythrocytes: hemolytic anemia & hemoglobinuria
Beta, epsilon & iota – necrosis
Enzymes – collagenase, hyaluronidase – spread of infection
Cl. perfringens: Pathogenicity
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Soft tissue infection: Gas gangrene (Type A)
Enteric infections:
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Food poisoning (some strains of type A)
Gangrenous appendicitis (A + D)
Necrotising enteritis (C)
Biliary tract infections
Other diseases:
– Brain abscess & meningitis
– Thoracic infections
Gas Gangrene
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Clostridium perfringens most frequent
clostridia involved in soft tissue and wound
infections – myonecrosis
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Spores found in soil, human skin, intestine,
and vagina
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Predisposing factors – crushing injuries, road
accidents, gunshot wounds, surgical
incisions, compound fractures, diabetic
ulcers, septic abortions, puncture wounds
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C/F - high fever, brownish pus, gas bubbles
under the skin, skin discoloration, and a foul
odor
Cl. tetani
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Gram +ve bacilli with terminal, spherical spore
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Commonly present in soil, GI tracts of animals
and inanimate objects like street & hospital
dust, cotton wool, bandages, catgut, wall
plaster, clothings, etc
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Causes tetanus or lockjaw, a neuromuscular
disease
– Most commonly among geriatric patients and
IV drug abusers; neonates in developing
countries
– Death commonly occurs due to paralysis of
respiratory muscles
Pathogenesis of Tetanus
Source of Infection
Contaminated soil / dust
Mode of transmission
Injury, puncture wounds, wound contamination
Unhygienic Complication
Septic abortion, Cutting umbilical stump
Ear boring / circumcision / unsterile injections
Virulence factor
Tetanospasmin – Neurotoxin
Tetanolysin
The events in tetanus
Spastic Paralysis in Tetanus
Ophisthotonus
Clostridium botulinum
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Gram +ve bacilli
with subterminal, oval spores
Produces toxin – Botulin
(botulinum toxin), a neurotoxin
Causes Botulism, a paralytic
(flaccid paralysis) disease
– Ingestion of preformed toxins
– In preserved (canned) food like meat
& meat products, fish & canned
vegetables
Effects of botulinum toxin
Phase I/ Module VII
Dr Ekta Chourasia
Clostridium difficile
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Normal resident of colon, in low
numbers
Causes antibiotic-associated colitis
(pseudomembranous colitis)
– Relatively non-invasive; treatment with
broad-spectrum antibiotics kills the
other bacteria, allowing C. difficile to
overgrow
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Produces enterotoxins & cytotoxins
that damage intestine
Major cause of diarrhea in hospitals
Non spore Forming
Gram Positive Bacilli
Gram-Positive
Non-Spore-Forming Bacilli
Medically important genera:
 Corynebacterium
 Listeria
 Propionibacterium
 Mycobacterium
 Actinomyces
 Nocardia
General Features of Corynebacteria
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Infects nasopharynx or skin
Mostly grow aerobically, nonmotile
Irregular swellings/clubbedshaped; granules within rod
stained with aniline dyes.
Corynebacterium diphtheriae
 Corynebacterium diphtheriae is the pathogen
responsible for diphtheria.
 It colonizes the pharynx, forming a grayish
pseudomembrane composed of fibrin, leukocytes,
necrotic epithelial cells, and Corynebacterium
diphtheriae cells.
 From this site, the bacteria release a powerful exotoxin
into the bloodstream, which specifically damages heart
and neural cells by interfering with protein synthesis.
Corynebacterium diphtheriae
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Blood agar+potassium tellurite, tellurite is reduced
intracellularly
C. diphtheriae - Virulence factor
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Exotoxin – Diphtheria toxin:
– Protein in nature
– very powerful toxin
– Responsible for all pathogenic effects of
the bacilli
– Produced by all the virulent strains
– Two fragments A & B
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Toxigenicity can be induced by
Lysogenic or phage conversion –
corynephages (tox+ phage) or beta
phages
Diphtheria toxin – Mechanism of action
DT - Acts by inhibition of protein synthesis
Fragment A – inhibits polypeptide chain elongation by inactivating
the Elongation factor EF 2 in the presence of NAD
Treatment
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Tracheostomy in children (to prevent croup)
Antitoxin
20000-100000 unit (Intra muscular)
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Penicillin or erythromycin
Prevention
Vaccination
A combination of diphtheria toxoid, tetanus toxoid, and
pertussis.
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Given at 2, 4 an 6 months of age, with a booster at 1 and 6
years of age and then each 10 years afterward. (DPT or
DT)
The toxoid is prepared by treating the exotoxin with 0.3%
formaldehyde.
Listeria monocytogenes
Small/Short, G+,peritrichous flagella, rod. Grow on
Mueller-Hinton agar (better in sheep blood-small zone
of haemolysis); facultative anaerobe, motile at room
temp, catalase +
 Listeriosis results from ingestion of contaminated food
such as cheese and vegie; primarily affects pregnant
women, newborns, those with weakened immune
system.
 Able to multiply at low temp,
hence accumulate in contaminated food stored in refrigerator.
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Listeria monocytogenes
It is a common cause of meningitis in 2 particular
groups:
 The first group is neonates, who contract this
organism from their asymptomatic mothers during
delivery.
 The second group of patients at risk for Listeria
meningitis is immunosuppressed patients, such as
those with cancer, renal transplants, or AIDS. The
mortality rate for meningitis in this second group is
extremely high.
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Steps in the invasion of cells and intracellular spread by L. monocytogenes.
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The bacterium apparently invades via the intestinal mucosa. It is thought to attach to intestinal cells by means of
D-galactose residues on the bacterial surface which adhere to D-galactose receptors on susceptible intestinal
cells The bacterium is taken up by induced phagocytosis, which is thought to be mediated by a membrane
associated protein called internalin. Once ingested the bacterium produces listeriolysin to escape from the
phagosome. The bacterium then multiplies rapidly in the cytoplasm and moves through the cytoplasm to invade
adjacent cells by polymerizing actin to form long tails.
Treatment
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When meningitis develops in a patient who is at
high risk for Listeria monocytogenes, it is important
to treat it empirically with antibiotics that will cover
this bacterium.
we must add either ampicillin or trimethoprimsulfamethoxazole to the antibiotic regimen. These
are 2 antibiotics that cover Listeria monocytogenes.
Genus Propionibacterium
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Propionibacterium acnes most common
Aerotolerant or anaerobic
Nontoxigenic
Common resident of pilosebaceous glands
Causes Acne
Mycobacteria: Acid-Fast Bacilli
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Gram-positive irregular bacilli
Acid-fast staining
Obligate aerobes
Possess mycolic acids and a
unique type of peptidoglycan
Do not form capsules, flagella,
or spores
Slow growing, 14-15 hrs
generation time
Mycobacterium Tuberculosis
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Produces no exotoxins or enzymes that
contribute to infectiousness
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Virulence factors – contain complex waxes and
cord factor that prevent destruction by
lysosomes or macrophages
Mycobacterium Leprae: The Leprosy
Bacillus
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Hansen’s bacillus/Hansen’s Disease
First bacterium to be shown to cause
disease in humans
Humans & Armadillos are the only
natural hosts
Strict parasite – cannot be cultivated “invitro” , does not grow on artificial media
or tissue culture
Slowest growing of all species
Causes leprosy, a chronic disease that
begins in the skin and mucous
membranes and progresses into nerves
Multiplies within host cells in large
packets called globi
Actinomycetes: Filamentous Bacilli
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Genera Actinomyces & Nocardia are
nonmotile filamentous bacteria
related to mycobacteria
May cause chronic infection of skin
and soft tissues
Actinomyces israelii – responsible
for diseases of the oral cavity,
thoracic or intestines –
actinomycoses “Sun-ray
appearnce”
Nocardia brasiliensis causes
pulmonary disease similar to TB