2. bacterial pathogenesis&lab diagnosis

Bacterial pathogenesis&

Laboratory diagnosis

By Walter Waswa,BSC.MLS, MSC.PUBLIC HEALTH

WALTER WAKHUNGU WASWA

Pathogenesis• The word comes from the Greek pathos, "disease",

and genesis, creation.

• The term pathogenesis means step by step development of a disease.

• The chain of events leading to that disease due to a series of changes in the structure and /or function of a cell/tissue/organ.

• Caused by a microbial, chemical or physical agent.


http://en.wikipedia.org/wiki/Greek_language

Pathogenicity - Ability to cause disease

Virulence - Degree of pathogenicity

• Many properties that determine a microbe’s pathogenicity or virulence are unclear or unknown

• But, when a microbe overpowers the hosts defenses, disease results!


Portals of Entry

• 1. Mucus Membranes

• 2. Skin

• 3. Parenteral


1. Mucus Membranes• A. Respiratory Tract

• Sticky mucous secretion

• Muco polysachharides

• Cough reflex

• Phagocytic cells


Common Diseases contracted via the Respiratory Tract

• Common cold

• Flu

• Tuberculosis

• Whooping cough

• Pneumonia

• Measles

• Strep Throat

• Diphtheria


Mucus Membranes

• B. Gastrointestinal Tract– Microbes gain entrance thru

contaminated food & water or fingers & hands.

– Most microbes that enter the G.I. Tract are destroyed by HCL & enzymes of stomach or bile & enzymes of small intestine.


Common diseases contracted via the G.I. Tract• Salmonellosis

– Salmonella sp.

• Shigellosis

– Shigella sp.

• Cholera

– Vibrio cholorea

• Ulcers

– Helicobacter pylori

• Botulism

– Clostridium botulinum


Fecal - Oral Diseases

• These pathogens enter the G.I. Tract at one end and exit at the other end.

• Spread by contaminated hands & fingers or contaminated food & water

• Poor personal hygiene.


Mucus Membranes of the Genitourinary System

Acidic urine

Vaginal secretion- Acidic due to fermentation of glycogen by

lactobacillus

Gonorrhea

Neisseria gonorrhoeae

Syphilis

Treponema pallidum

Chlamydia

Chlamydia trachomatis

HIV

Herpes Simplex IIWALTER WAKHUNGU WASWA

Mucus Membranes

• D. Conjunctiva –

Lacrymal secretion

• Lysozyme

• Trachoma

– Chlamydia trachomatis


2. Skin

• Skin - the largest organ of the body.

• Sebaceous secretion-containing fatty acids

• Resident flora

• When unbroken is an effective barrier for most microorganisms.

• Some microbes can gain entrance thru openings in the skin: hair follicles and sweat glands


3. Parentarel

• Microorganisms are deposited into the tissues below the skin or mucus membranes

• Punctures

• injections

• bites

• scratches

• surgery

• splitting of skin due to swelling or dryness


Number of Invading Microbes

• LD50 - Lethal Dose of a microbes toxin that will kill 50% of experimentally inoculated test animal

• ID50 - infectious dose required to cause disease in 50% of inoculated test animals– Example: ID50 for Vibrio cholerea 108 cells

(100,000,000 cells)

– ID50 for Inhalation Anthrax - 5,000 to 10,000 spores


How do Bacterial Pathogens

penetrate Host Defenses?

1. Adherence - almost all pathogens

have a means to attach to host tissue

Binding Sites

adhesins

ligands


How Bacterial Pathogens Penetrate Host Defenses

• 1. Adherence

• 2. Capsule

• 3. Enzymes

– A. leukocidins

– B. Hemolysins

– C. Coagulase

– D. Kinases

– E. Hyaluronidase

– F. Collagenase

– G. Necrotizing FactorWALTER WAKHUNGU WASWA

Adhesins and ligands are usually on Fimbriae

• Neisseria gonorrhoeae

• ETEC (Entertoxigenic E. coli)

• Bordetella pertussis


2. Capsules• Prevent phagocytosis

• Attachment

• Streptococcus pneumoniae

• Klebsiella pneumoniae

• Haemophilus influenzae

• Bacillus anthracis

• Streptococcus mutans

• Yersinia pestis

K. pneumoniae WALTER WAKHUNGU WASWA

3. Enzymes

• Many pathogens secrete enzymes that contribute to their pathogenicity


A. Leukocidins

• Attack certain types of WBC’s

• 1. Kills WBC’s which prevents phagocytosis

• 2. Releases & ruptures lysosomes

– lysosomes - contain powerful hydrolytic enzymes which then cause more tissue damage


B. Hemolysins - cause the lysis of RBC’s

Streptococci


1. Alpha Hemolytic Streptococci

- secrete hemolysins that cause the

incomplete lysis or RBC’s


2. Beta Hemolytic Streptococci

- Secrete hemolysins that cause the complete lysis of RBC’s


C. Coagulase - cause blood to coagulate

• Blood clots protect bacteria from phagocytosis from WBC’s and other host defenses

• Staphylococci - are often coagulase positive

– boils

– abscesses


D. Kinases - enzymes that dissolve blood clots

• 1. Streptokinase - Streptococci

• 2. Staphylokinase - Staphylococci

• Helps to spread bacteria - Bacteremia

• Streptokinase - used to dissolve blood clots in the Heart (Heart Attacks due to obstructed coronary blood vessels)


E. Hyaluronidase

• Breaks down Hyaluronic acid (found in connective

tissues)

• “Spreading Factor”

• Mixed with a drug to help spread the drug through a body tissue


F. Collagenase

• Breaks down collagen (found in many connective

tissues)

• Clostridium perfringens - Gas Gangrene

– uses this to spread thru muscle tissue


G. Necrotizing Factor

- Causes death (necrosis) to tissue cells

“Flesh Eating Bacteria”


Bacterial Toxins• Poisonous substances produced by

microorganisms

• Toxins - primary factor - pathogenicity

• 220 known bacterial toxins

– 40% cause disease by damaging the Eukaryotic cell membrane

• Toxemia

– Toxins in the bloodstream


Types of Toxins• 1. Exotoxins

– Exotoxins are generated by the bacteria and actively secreted

– Secreted outside the bacterial cell

• 2. Endotoxins

– Part of the outer cell wall of Gram (-) bacteria

– The body's response to endotoxin can involve severe inflammation.


Exotoxins• Mostly seen in Gram (+) Bacteria

• Heat labile, can be inactivated by heating at 60-80 ۠ C.

• Excreted [ secreted] from the microbial cells into the surrounding ie culture media or circulatory system

• Don’t require bacterial death or cell lysis for their release.


Types of Exotoxins

• 1. Cytotoxins

– Kill cells- shigella, vibrio

• 2. Neurotoxins

– Interfere with normal nerve impulses

– Clostridium botulinum

– Clostridium tetani

• 3. Enterotoxins

– Effect cells lining the G.I. Tract

– E. coli

– Salmonella WALTER WAKHUNGU WASWA

Response to Toxins

• If exposed to exotoxins: antibodies against the toxin (antitoxins)

• Exotoxins inactivated ( heat, formalin or phenol) no longer cause disease, but stimulate the production of antitoxin

– altered exotoxins - Toxoids

• Toxoids - injected to stimulate the production of antitoxins and provide immunity


Endotoxins • Part of the Gram (-) Bacterial cell wall.

• Biological activity or toxicity of endotoxin is largely due to lipid A.

• Relatively heat stable can withstand heat over 60 ۠

C for many hours.

• Released upon cell lysis or death.

• Less potent than exotoxins, active in large doses only.

• Pyrogenic often produce fever in hosts.

• Salmonella, Shigella, Escherichia, Neisseria.WALTER WAKHUNGU WASWA

Bacteriocin• Bacteriocins were first discovered by A. Gratia in 1925.

• He called his first discovery a colicine because it killed E. coli.

• Bacteriocins are proteinaceous toxins produced by bacteria to inhibit the growth of similar or closely related bacterial strain(s).

• They are typically considered to be narrow spectrum antibiotics, though this has been debated.


• Medical significance

• Bacteriocins are of interest in medicine because they are made by non-pathogenic bacteria that normally colonize the human body.

• Loss of these harmless bacteria following antibiotic use may allow opportunistic pathogenic bacteria to invade the human body.


• Manifestations of Infection: Signs and symptoms vary according to the site and severity of infection. Diagnosis requires a composite of information, including history, physical examination, radiographic findings, and laboratory data.

• Microbial Causes of Infection: Infections may be caused by bacteria, viruses, fungi, and parasites. The pathogen may be exogenous (acquired from environmental or animal sources or from other persons) or endogenous (from the normal flora).


Specimen Selection, Collection, and Processing

• The quantity material must be adequate

• Specimens are selected on the basis of signs and symptoms, should be representative of the disease process

• Contamination of the specimen must be avoided by using only sterile equipment and aseptic precautions

• The specimen must be taken to the laboratory and examined promptly. Special transport media may be helpful.

• Meaningful specimens to diagnose bacterial infections must be secured before antimicrobial drugs are administered.WALTER WAKHUNGU WASWA

Microbiologic Examination

• Culture:Isolation of infectious agents frequently requires specialized media. Nonselective (noninhibitory) media permit the growth of many microorganisms. Selective media contain inhibitory substances that permit the isolation of specific types of microorganisms.

• Microbial Identification: Colony and cellular morphology may permit preliminary identification. Growth characteristics under various conditions, utilization of carbohydrates and other substrates, enzymatic activity, immunoassays, and genetic probes are also used.

• Antimicrobial Susceptibility: Microorganisms, particularly bacteria, are tested in vitro to determine whether they are susceptible to antimicrobial agents.

• Serodiagnosis:A high or rising titer of specific IgG antibodies or the presence of specific IgM antibodies may suggest or confirm a diagnosis.

• Direct Examination and Techniques: Direct examination of specimens reveals gross pathology. Microscopy may identify microorganisms. Immunofluorescence, immuno-peroxidase staining, and other immunoassays may detect specific microbial antigens. Genetic probes identify genus- or species-specific DNA or RNA sequences.


General procedure for collecting and processing specimens for

aerobic and/or anaerobic bacterial culture


Agglutination test in which inert particles (latex beads or heat-killed S aureus Cowan

1 strain with protein A) are coated with antibody to any of a variety of antigens and

then used to detect the antigen in specimens or in isolated bacteria.WALTER WAKHUNGU WASWA

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2. bacterial pathogenesis&lab diagnosis

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