Medical Microbiology EDPT 211 Past Paper PDF

Kingdom of Saudi Arabia ‫اﻟﻣﻣﻠﻛﺔ اﻟﻌرﺑﯾﺔ اﻟﺳﻌودﯾﺔ‬ Ministry of Education ‫وزارة اﻟﺗﻌﻠﯾم‬ Qassim University ‫ﺟﺎﻣﻌﺔ اﻟﻘﺻﯾم‬ College of Medical Rehabilitati...

Kingdom of Saudi Arabia ‫اﻟﻣﻣﻠﻛﺔ اﻟﻌرﺑﯾﺔ اﻟﺳﻌودﯾﺔ‬ Ministry of Education ‫وزارة اﻟﺗﻌﻠﯾم‬ Qassim University ‫ﺟﺎﻣﻌﺔ اﻟﻘﺻﯾم‬ College of Medical Rehabilitation ‫ﻛﻠﯾﺔ اﻟﺗﺎھﯾل اﻟطﺑﻰ‬ Medical Microbiology [EDPT 211] Prof. Dr. Noha Mohammed Afifi Chapter 7 Host-Parasite Relationship Bacterial Nutrition HETEROTROPHIC: Bacteria obtain food made by other organisms: (I) Saprophytic bacteria: bacteria live freely in nature and obtain food from dead or decaying organic matter (dead leaves) e.g. decomposer bacteria in soil. They do NOT require a living host. (II) Parasitic bacteria: bacteria obtain nutrients from living host, causing them harm. The parasite obtains nutrients by living in or on the body of the host, e.g. disease-causing (Pathogenic Bacteria).  A Microorganism is Pathogen: if it is capable of causing disease. Prof. Noha Afifi 48  Parasitic Bacteria are classified according to their relation to the host into: a. Pathogenic (Parasites): Parasitic bacteria that can multiply in the host tissue causing disease. b. Non-Pathogenic (Commensals): Bacteria that live on the external or internal surfaces of the body without causing disease. They constitute the largest group of Normal body Flora. They may be even beneficial to the host; Relation is “Mutalism” e.g.  Commensals in the intestine digest polysaccharides and produce certain vitamins as (Vitamin B & K).  Normal flora protect against harmful pathogens as they Competitively exclude Pathogens through: - - Covering binding sites thus preventing pathogen attachment & colonization. - Competing with pathogenic organisms for nutrition, thus inhibiting their growth. - Producing toxic substances, e.g. Lactobacilli in the vagina produce Lactic acid that inhibits growth of: a) the pathogenic bacteria that causes “Bacterial Vaginosis” and also b) the Fungus Candida albicans. [Inactivation of Lactobacilli is caused by excessive Vaginal Douching by soap & other Chemicals and also Smoking]. c. Opportunistic Pathogens: Commensal bacteria that can cause disease under certain conditions & are considered as "Potential Pathogens" or "Opportunists". Some of these Conditions are: 1- Lowered host defense mechanisms: Immune-compromised e.g. diabetics, HIV (AIDS patients) or patients receiving immunosuppressive drugs as Cortisone ----or Chemotherapy. Prof. Noha Afifi 49 2- Change in the natural habitat of the organisms e.g. a) E. Coli, is a normal inhabitant of the intestine. If it reaches the urinary system; it causes urinary tract infection (UTI). b) Streptococcus Viridans, a normal inhabitant of the mouth; may cause Endocarditis if they get access to the blood (bacteremia) after tooth extraction (if the host has a predisposing heart lesion). Infection is Defined as: The process by which the parasite enters into a relationship with the host; achieved by Entrance and multiplication of the microorganism in the host. Disease is Defined as: Disease is defined as: Damage of the host tissue due to invasion or toxin production by microorganisms. Pathogenesis of Bacterial Infection requires certain steps called the “Infectious Cycle” Steps of the Infectious Cycle: 1) Source of Infection: Either a “Case or Carrier”.  Case: Person harboring the organism & presenting with clinical signs and symptoms. Case is either: * Man * Animal (Zoonosis) e.g. Rabies by dogs, Anthrax & T.B. in milk of infected animals) or -* Arthropod (e.g. mosquito transmitting malaria). Prof. Noha Afifi 50   Carrier. Carriers  A Carrier is an apparently healthy individual who  Carries a pathogenic organism  Without showing clinical manifestations &  Can transmit this organism to other susceptible individuals. Types of Carriers:  Transient carrier during the incubation period or  Chronic carrier e.g. Hepatitis B or C. Examples of Carriers: - Intestinal (Gall Bladder) carriers: e.g. Salmonella. - Nasopharyngeal carriers: Neisseria Meningitides. - Throat (Oropharyngeal) carriers: Diphtheria. - Nasal / Skin carriers: Staphylococcus aureus. - Blood carriers; HIV, HBV, HCV. Carriers are more dangerous than cases as a source of infection BECAUSE: 1. They are not known to public. 2. They are not easily detected. 3. They are not restricted to bed. 2) Mode of Transmission to New Host: - Contact Transmission: (Skin-to-skin contact by Touching, Kissing) or Sexual Intercourse (e.g. AIDS, Syphilis, Gonorrhea, Herpes). - Droplet Infection: Microbes spread in droplets that travel short distance (less than 1 meter) (e.g. Tonsillitis, Diphtheria, Meningitis, Pneumonia, Whooping cough). - Air-borne Transmission: Microbes spread by droplets in dust that travel long distance (more than 1 meter) to the host (e.g. T.B., Measles, Chicken Pox). Prof. Noha Afifi 51  - Vector Transmission: Insect bite (e.g. Malaria). - Blood-born Transmission: Blood transfusion or injection (Parenteral) e.g. (Hepatitis B, C and D and HIV of AIDS). - Vertical Transmission): from infected mother to fetus or baby during pregnancy (Transplacental), childbirth or lactation e.g. AIDS (HIV), Syphilis & Rubella. - Fecal-oral e.g. Typhoid Fever, Dysentery & Hepatitis A. 3) Portal of Entrance of the parasite into the new host: Most pathogens have Specific Portals of entry: - Skin and mucous membranes: through abrasions, insect bites or injections (Parenteral). - Gastro-intestinal Tract (GIT) [Ingestion]. - Respiratory Tract (mouth and nose) [Inhalation] - Genito-urinary Tract; Sexually-Transmitted Diseases (STDs) [Sexual Intercourse]. 4) Multiplication of the parasite within the susceptible host, causing Tissue Damage: The parasite may multiply locally at the portal of entry or it may spread through the tissues, blood or lymphatics to reach a target organ. For the organism to be propagated in nature, it should have: 5) Portal of exit from the host: in secretions e.g. in urine, stools, respiratory or genital discharges, blood. Factors that Govern Disease Production Only few infections end in “Clinically manifest disease”. Most infections are abortive, silent or subclinical. The outcome of infection depends on interaction between 1- Microbial Factors (Pathogenicity and Virulence). 2- Host Resistance Factors (Natural and Acquired immunity). 52  1- Microbial Factors  Pathogenicity: It is the ability of an organism to cause disease (Qualitative).  Virulence: The severity of the disease produced by the organism (Quantitative) - It is the degree of pathogenicity of a strain belonging to a pathogenic species. - In a pathogenic species of bacteria e.g. C. Diphteria:  Some strains are highly virulent producing severe disease.  Others are moderately virulent producing mild disease.  While others are avirulent i.e. unable to produce disease. This depends on the ability of different strains to produce a TOXIN and on its potency (acquired by Prophage). Prof. Noha Afifi Virulence is expressed as LD50 i.e. lethal dose (the number of organisms) or [micrograms of toxin] needed to kill 50% of the animals. Virulence Factors of Bacteria Either a structure or a product that help microorganisms to overcome body defense mechanisms and cause disease. 1) Adherence Factors: o Certain bacteria have specialized structures that help their adhesion to host cell mucous membranes, thus allow them to start the disease process (Colonization). o Mutants that lack these structures are often Avirulent. o For example: the fimbriae/pili of Neisseria Gonorrhea & E. Coli mediate the attachment of the organisms to the urinary tract epithelium. Prof. Noha Afifi 53  2) Invasion Factors: o This is the ability to invade tissues, multiply rapidly causing the inflammatory process. o This may be partly due to the Antiphagocytic action of bacterial capsule that protects the bacteria from phagocytosis and destruction e.g. the polysaccharide capsule of Pneumococci. 3) Extracellular Enzymes: o These are substances produced by some bacteria that help the spread, invasion and establishment of microorganisms into the tissues. These include: a) Collagenase: It breaks down collagen fibers and promotes spread of infection. b) Hyaluronidase: splits hyaluronic acid; a constituent of the cement substance of connective tissue. It helps spread of infection and is produced by Streptococci. c) Lecithinase: It breaks down lecithin a constituent of cell membrane. d) Immunoglobulin A (IgA) Protease: It degrades the secretory IgA on mucous surfaces & thus the host loses protection by this antibody (IgA). e) Coagulase: is produced by Staphylococcus. aureus. It coagulates fibrinogen in plasma into fibrin, with deposition of a fibrin wall around Staphylococcal lesions which helps them to form localized infections (e.g. abscess), and protecting them from phagocytosis, antibodies and antibiotics. f) Fibrinolysin (Streptokinase): is produced by Streptococcus pyogenes. In microrganisms, it causes lysis of fibrin clots, at the site of infection, helping spread of organisms in tissues (e.g. Cellulitis). In therapy, it is given as injection to dissolve thrombi in coronary artery (Thrombolytic Agent). h) DNase: Degrade extracellular DNA to help spread of the organism. Prof. Noha Afifi 54  4) Toxin Production:  Toxins are bacterial products which have a direct harmful action on tissue cells.  They fall into 2 groups: a- Exotoxins & b- Endotoxins. Exotoxins Endotoxins - - They are integral part of the cell - - Produced by living pathogenic Source bacteria, most commonly Gram +ve wall of Gram negative bacteria bacteria. (Lipid A portion of LPS) from which Diffusibility - - Diffuse freely into the surrounding they are liberated, when the bacteria medium i.e. Extracellular toxins. dies and the cell wall breaks apart. - - No diffusibility. Chemical Protein Lipid A Fraction of Composition Lipopolysaccharide (LPS) Genes controlling Located on Plasmid or Located on bacterial production Bacteriophage Chromosome Antigenicity Highly Antigenic (induce high titre Weakly antigenic of anti-toxin) Symptoms - - Act specifically on target cells (e.g. - All endotoxins produce Produced muscle, nerve) producing specific generalized Non-specific toxic symptoms effect of: fever, hypotension, haemorrhage, DIC and necrosis of internal organs & sometimes death due to multiple organ failure. This is known as Endotoxic Shock. Heat Stability Heat labile (loses toxicity at 60 ᴼc) Heat Stable at 100°c for 1 hour 55 Toxoid Can be prepared & used as vaccine Can not be prepared. (inactive Toxin) No vaccine production. Produced by Mainly Gram positive Gram negative bacteria bacteria…..C. diptheriae & …Salmonella, Shigella, Clostridium tetani & V. cholera & E. Coli Clostridium botulinum Toxoid is: - A toxoid is a substance that is normally toxic. - It is treated with formalin, so that it loses toxicity, but retains antigenicity (capacity to produce antibody by the immune system) - These "detoxified toxins" are called "Toxoids" and are used for vaccination. - Examples of Toxoid-based vaccines include: Tetanus & Diphtheria. Prof. Noha Afifi 56