2. :
Medical microbiology
the field of microbiology that deal with study of pathogenic microorganisms
and their interactions with human as causative agents of infectious diseases,
with the prevention, diagnosis and treatment of these diseases
.
It covers the branches of bacteriology, virology, mycology, parasitology and
immunology
.
:
Microorganisms/microbes
are microscopic living organisms which are very diverse and invisible by naked
eye. They can be found in nearly every environment. The life in our planet is
impossible without microbes due to their important role in generation of
energy.
:
Types of microbial pathogens
The pathogenic organisms of infectious diseases can be classified into four
major groups of organisms : viruses, bacteria , fungi and parasites. Differences
( between these groups are listed in table number 1)
3. Staphylococcus or Staphylococcus :
is a genus of bacteria in the family Staphylococcus, its description comes from
the Greek, roughly meaning grape pellets. Staphylococci are non-motile, non-
spore-forming cocci, usually arranged in clusters, positively staining by Gram
method, and generally facultatively anaerobic.
Scientific name: Staphylococcus
Family: Staphylococcus
Staphylococcus
is the most dangerous of all the different
aureus : and common types of Staphylococcus.
Among its risks:
1)- This bacteria is spread by direct contact with an infected person, an object
contaminated with the bacteria, or by inhaling cough or sneezing droplets
containing the bacteria.
2)- Skin infections are common, but the bacteria can spread through the
bloodstream and infect distant organs.
3)- Skin infections may cause pustules, abscesses, and redness and swelling in
the affected area.
4)- People who transfer bacteria from their nose to other parts of the body with
their hands, sometimes resulting in infection.
Q: Who is most susceptible to Staphylococcal infection?
While anyone can get a staph infection, some conditions put people at greater
risk including: -Newborn babies. - Breastfeeding women. -Diabetics. -Diseases
of the blood vessels or lung. -cancer. -Weakened immune system. - skin
injuries.
4. A person becomes infected with this type of bacteria in one of the following
ways:
(1)- skin infection:
cellulitis ،boils ،herpes ،Staphylococcal scalded skin ،syndrome.
(2)-food poisoning:
Staphylococcus bacteria is one of the most common causes of food poisoning.
Symptoms appear quickly, usually within a few hours of eating contaminated
food. Symptoms also usually go away quickly, often lasting only half a day.
(3)-bacteremia:
It occurs when staph bacteria enter a person's bloodstream. Fever and low
blood pressure are signs of bacteremia. The bacteria can travel deep inside your
body to produce an infection that affects:
-Internal organs, such as your brain, heart, or lungs.
- Bones and muscles.
- Surgically implanted devices, such as artificial joints or pacemakers.
(4)-toxic shock syndrome:
- This potentially fatal condition is caused by toxins produced by some strains of
staph bacteria, and has been linked to certain types of tampons, skin wounds
and surgery.
5. STRUCTURE
Staphylococci are Gram-positive cocci 1μm in diameter. They form clumps .
Classification
S aureus and S intermedius are coagulase positive. All other staphylococci are coagulase
negative. They are salt tolerant and often hemolytic. Identification requires biotype analysis.
Natural Habitat
S aureus colonizes the nasal passage and axillae. S epidermidis is a common human skin
commensal. Other species of staphylococci are infrequent human commensals. Some are
commensals of other animals.
Pathogenesis
S aureus expresses many potential virulence factors. (1) Surface proteins that promote
colonization of host tissues. (2) Factors that probably inhibit phagocytosis (capsule,
immunoglobulin binding protein A). (3) Toxins that damage host tissues and cause disease
symptoms. Coagulase-negative staphylococci are normally less virulent and express fewer
virulence factors. S epidermidis readily colonizes implanted device
Host Defenses
Phagocytosis is the major mechanism for combatting staphylococcal infection. Antibodies are
produced which neutralize toxins and promote opsonization. The capsule and protein A may
interfere with phagocytosis. Biofilm growth on implants is impervious to phagocytosis.
Treatment
Infections acquired outside hospitals can usually be treated with penicillinase-resistant β-lactams.
Hospital acquired infection is often caused by antibiotic resistant strains and can only be treated
with vancomycin.
6. Reference
1. Bhakdi S, Tranum-Jensen J. Alpha-toxin of Staphylococcus aureus.
Microbiol Rev. 1991;55:733. [PMC free article] [PubMed]
2. Easmon CSF, Adlam C: Staphylococci and staphylococcal infections. Vols 1
and 2. Academic Press, London, 1983 .
3. Foster TJ. Potential for vaccination against infections caused by
Staphylococcus aureus. Vaccine. 1991;9:221. [PubMed]
4. Foster TJ, McDevitt D: Molecular basis of adherence of staphylococci to
biomaterials. p. 31, In Bisno AL, Waldvogel FA (eds): Infections Associated
with Indwelling Medical Devices, 2nd Edition. American Society for
Microbiology, Washington, D.C., 1994.
5. Lyon BR, Skurray R. Antimicrobial resistance in Staphylococcus aureus:
genetic basis. Microbiol Reviews. 1987;51:88. [PMC free article] [PubMed]
6. Prevost G, Couppie P, Prevost P. et al. Epidemiological data on
Staphylococcus aureus strains producing synergohymenotropic toxins. J
Med Microbiol. 1995;42:237. [PubMed]
7. Rupp ME, Archer GL. Coagulase-negative staphylococci: pathogens
associated with medical progress. Clin Infect Dis. 1994;19:231. [PubMed]
8. Schlievert PM. Role of superantigens in human disease. J Infect Dis.
1993;167:997. [PubMed]
9. Skinner GRB, Ahmad, A: Staphylococcal vaccines - present status and
future prospects. p. 537. In Mollby R, Flock JI, Nord CE, Christensson B
(eds): Staphylococci and Staphylococcal Infections. Zbl. Bakt. Suppl. 26,
Fischer Verlag, Stuttgart, 1994 .
10. Tenover F, Arbeit R, Archer G. et al. Comparison of traditional and
molecular methods of typing isolates of Staphylococcus aureus. J Clin Microbiol.
1994;32:407. [PMC free article] [PubMed]
11. Vaudaux PE, Lew DP, Waldvogel FA: Host factors predisposing to and
influencing therapy of foreign body infections. p. 1. In Bisno AL, Waldvogel FA
(eds): Infections Associated with Indwelling Medical Devices. 2nd Ed. American
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