Download There are four mainly important genera of gram positive rods

Clinical Microbiology
MLCM- 201)
Prof. Dr. Ebtisam. F. El Ghazzawi.
Medical Research Institute (MRI)
Alexandria University
Intended Learning Outcomes
By the end of this lecture the student should be able to
understand all about Gram Positive Rods Types
pathogens and there Laboratory Diagnostic tests.
There are four mainly important genera of
gram positive rods: bacillus, clostridium,
corynebacterium and listeria.
Bacillus and clostridium form spores,
whereas corynebacterium and listeria do
not. Members of the genus bacillus are
aerobic, whereas those of the genus
clostridium are anearobic.
The
gram-positive
rods
can
also
be
distinguished based on their appearance on
gram stain.
Bacillus and clostridium species are longer
and more deeply staining than those of
corynebacterium
and
listeria
species.
Corynebacterium species are club-shaped,
i.e. they are thinner on one end than the
other. Corynebacterium and listeria species
characteristically appear as V or L-shaped
rods.
Many important infections due to bacteria are
common.
The infections are often polymicrobial, that is,
the anaerobic bacteria are found in mixed
infections with other anaerobes, facultative
anaerobes and aerobes. Anaerobic bacteria
are found throughout the human body, on the
skin, on mucosal surfaces and in high
concentrations
in
the
mouth
and
gastrointestinal tract, as part of the normal
flora.
Infection results when anaerobes and other
bacteria of the normal flora contaminate
normally sterile body sites.
Several important diseases are caused by
anaerobic Clostridium species from the
environment or from normal flora: botulism,
tetanus, gas gangrene, food poisoning and
pseudomembranous colitis.
There are four medically important species:
Cl. tetani, Cl. botulinum, Cl. perfringens
(which causes either gas gangrene or food
poisoning), and Cl. difficili. All Clostridia are
anaerobic, spore forming, gram-positive
rods.
Cl. tetani causes tetanus (lockjaw).
Spores are widespread in soil. The portal of
entry is usually wound site. e.g. where a nail
penetrates the foot. Germination of the
spores is favored by necrotic tissue and
poor blood supply in the wound.
Neonatal tetanus in which the organism
enters through a contaminated umbilicus or
circumcision wound is a major problem in
some
developing
countries.
Once
the
organism grow it elaborate a toxin that
spreads along nerves to the central nervous
system (CNS) and leads to muscle spasm.
Death results from inability to breath.
There is no microbiologic or serologic
diagnosis. Organisms are rarely isolated from
the wound site. Cl. tetani produces a terminal
spore i.e. spore at the end of the rod. This
gives
the
organism
the
appearance of a “tennis racket”.
characteristic
Cl. botulinum causes botulism.
Spores widespread in soil, contaminated
vegetables and meats. When these foods
are
canned
or
vacuum-packed
without
adequate sterilization, spores survive and
germinate in the anaerobic environment.
Toxin is produced within the canned food and
ingested performed. The highest risk foods
are:
1. Alkaline vegetables such as green beans,
peppers and mushrooms.
2. Smoked fish. The toxin is relatively heat
labile, it is inactivated by boiling for
several minutes. Thus, disease can be
prevented by sufficient cooking.
The organism is usually not cultured.
Botulism toxin is demonstrated by injecting
a mice with a sample of clinical specimen,
where it will dies unless protected by
antitoxin.
Cl. perfringens causes two distinct diseases,
gas gangrene and food poisoning.
Spores are located in the soil, vegetative cells
are members of the normal flora of the colon
and vagina. Gas gangrene is associated with
war wounds, automobile and motorcycle
accidents and septic abortions.
Pain, oedema and cellulitis occur in the
wound
area.
Crepitation
indicates
the
presence of gas in tissues. Hemolysis and
jaundice are common as are blood-tinged
exudates. Shock and death can occur.
Mortality rates are high.
Smears of tissue and exudate samples show
large gram-positive rods. The organisms are
cultured anaerobically. On blood agar Cl.
perfringens colonies exhibit a double zone of
hemolysis.
There are two important pathogens in this
group:
Corynebacterium
diphtheriae
and
Listeria monocytogenes.
C.
diphtheriae
causes
diphtheria.
Other
Corynebacterium species (diphtheroids) are
implicated in opportunistic infections.
Corynebacteria
are gram-positive rods that
appear club-shaped (wider at one end) and
are arranged in palisades or V or L-shaped
formation or Chinese like structure.
Humans are the only natural host of C.
diphtheriae. The organisms reside in the
upper respiratory tract and are transmitted by
airborne droplets.
The host response to C. diphtheriae consist
of:
1. Local inflammation in the throat with a
fibrinous exudate that forms the tough,
adherent, gray pseudomembrane over the
tonsils and throat characteristic of the
disease.
2. Antibody
activity.
that
can
neutralize
exotoxin
Involves both isolating the organism and
demonstrating
toxin
production.
The
decision to treat with antitoxin is a clinical
one and cannot wait for the laboratory
results. A throat swab should be cultured on
Löffler’s medium and blood agar.
Immunization of children by DPT (Diphtheria
pertusis and tetanus toxoids) in the age of 3,
4 and 6 months with a boosters at 1 and 6
years of age.
Assignment
Escherichia Coli.
Questions
Complete the following :
 There are four mainly important genera of gram
positive rods………..,…………,……….and…….
 Cl. tetani causes………(…………).
 Cl. botulinum causes …………….
Recommended Textbooks
Manual of Clinical Microbiology, Vols. 1 and 2:
Eighth Edition Patrick R. Murray