University Of Veterinary And Animal Sciences, Lahore
Microbiology
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A guide to Medical Microbiology: Your Essential Handbook for Acing Basic Exams
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Course
Microbiology
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University Of Veterinary And Animal Sciences, Lahore
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Medical Microbiology E-Book
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Microbiology
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STAPHYLOCOCCUS
Some species of staphylococci are:
• Staphylococcus aureus
• Staphylococcus epidermidis
• Staphylococcus saccharolyticus (previously known as S. saprophyticus)
Morphology:
• Gram-positive cocci, grow in clusters resembling bunches of grapes
• Capsulated
• Non-endospore forming
• Non-motile
Habitat:
• Staphylococcus species occur as commensals on the skin and mucous membranes
• Most abundant in the nares
• Also present in the environment
Route of Infection:
Staphylococci cause opportunistic infections, with S. aureus being the most pathogenic.
• Endogenous infections
Staphylococci present on or in the human body cause infections under the influence of
predisposing infections
• Exogenous infection
Staphylococci enter the individual’s body from the environment.
Predisposing factors
Predisposing factors are those that make an individual susceptible to a disease or put them at risk
for developing a particular disease. Pre-disposing factors differ with respect to the system.
SKIN INFECTION:
Pre-disposing factors for skin infections by staph aureus are as follows:
Physical factors
,high temperatures can cause burn wounds. For example, boiling water can damage the skin
making it more susceptible to infection.
Low temperature
skin can get damaged because of cold application
High osmotic pressure
swimmers can damage their skin barrier because of remaining in water for a long time.
Injury
wounds caused by accidents or surgery are a pre-disposing factor as well.
Chemicals
bleach, carbolic acid, and many other disinfectants used in high concentrations
Microbial infections
infections by herpes virus, pox virus, or retrovirus.
Exposure to radiation
X-rays, gamma rays, or UV rays
Other infections
Skin benign tumors can also be a factor.
Diabetes mellitus
Mechanisms to escape phagocytosis
• When a wound is inflicted, blood clots and forms a thrombus to which
staphylococcus aureus can attach. Staphylococcus escapes the clot/thrombus with
the help of the enzyme staphylokinase. This enzyme dissolves the fibrin clot.
• With the help of the capsule, staphylococcus shows gliding motility and escapes
phagocytosis.
• Staphylococcus aureus can also escape opsonization with the help of protein A.
Opsonins present in the blood include immunoglobulins, complement proteins,
and growth factors. They attach to microbial surfaces and help phagocytosis.
Protein A has an affinity for the Fc region of antibodies, by binding to this region
it renders them inaccessible to phagocytosis.
Virulence factors
1. Toxins
, 2. Enzymes
Toxins
Toxins are secondary metabolites. Some important toxins are discussed below:
1. Epidermolytic: it causes the lysis of epidermal cells. This toxin is the cause of scalded
body syndrome in which patches of discoloration are formed on the skin.
2. Hemolysins: In blood, four types of hemolysins are produced by S. aureus. Alpha, beta,
gamma, and delta. Some can synthesize alpha and beta simultaneously as well. They
show double hemolysis on blood agar and are the most pathogenic.
3. Leukoocidins: Leukocidins target WBCs. Pyrexia is observed. As a result of
immunosuppression secondary infections can also occur, leading to a complex disease.
4. TSST: Toxic shock syndrome toxin targets the blood vessels and dilates them. The
following physiological changes are observed:
Heart rate increases and the cardiac cells shift to anaerobic fermentation. Lactic acid is
deposited in the cardiac muscles. The patient feels heart pain. Because of muscle fatigue
heart rate decreases and blood doesn’t reach the extremities. The extremities turn cold. It can
lead to dizziness, hypnosis, pre-anesthesia, anesthesia, and death.
Enterotoxins: cause food poisoning.
Enzymes
Enzymes are primary metabolites.
1. Hyaluronidase: causes desquamation of skin cells. cells disintegrate and die
2. Coagulase: Coagulase in capillaries causes clotting. Blood supply to tissues is cut off
causing tissue necrosis. Necrotic tissue is seen as yellow in color.
Respiratory System:
Other than skin, Staphylococcus aureus’ major target is the respiratory system. More than 40%
of S. aureus is present in nares but normally doesn’t cause disease because of innate immunity
barriers. These are,
1- Hair: Removing nasal hair could impair this barrier and the wound could form on the
mucus membrane.
, 2- Cilia in the upper respiratory system: Cilia move in high synchronization. Microbes that
cross cilia couldn’t cross it due to ciliary movement.
3- Epiglottis: It stops the entry of particulate matter into the respiratory system.
4- Ciliary escalation: Microorganisms as well as particulate matter are trapped in
secretions i.e. mucus, and are removed by ciliary escalation.
5- Sneezing expels microbes from the body
6- Coughing also expels microbes
All these mechanisms are involved in immunity. Some predisposing factors for staphylococcal
infection are,
1- Genetic disorders cause immunosuppression e.g. teratogenicity, or body systems not
fully developed.
2- Viruses cause infections that lower immunity like HIV, Hepatitis, etc.
3- Respiratory system targeting viruses, for example, influenza virus or COVID can
damage the respiratory system
4- Bacterial infections also lower immunity for example tuberculosis and whooping cough
can damage the respiratory system
5- Parasitic diseases target the respiratory system for example lungworms
(nematode/roundworms) _ their larvae migrate towards the respiratory system to damage
it
6- Mycotic infections: fungi play important role in mycotic pneumonia which is caused by
aspergillus fumigatus and it damages the respiratory system
7- Hypersensitivity or allergic reactions: hypersensitivity type one or anaphylaxis or
anaphylactic reactions could be systemic or local reactions.
8- Stress factor: Environmental pollution is a stress factor targeting the respiratory system
for example living in an industrial area or vehicle emission or emission of carbon
particles or bricks formation or fire. These chemicals and carbon particles affect the
respiratory system.
9- Drug abusers: Most drugs are inhaled or based on volatile solvation (made from coal) so
their vapors can cause damage.
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