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Batterjee Medical College Batterjee Medical College Normal flora Dr. Manal El Said Head of Microbiology Department Batterjee Medical College Normal flora Normal flora (commensals) • They are those microorganisms that are permanent residents of body that everyone has. • Commensals are organisms that derive benefit from another host but do not damage that host. Batterjee Medical College Normal flora Normal flora organisms are: • Bacteria or yeasts (No Viruses, protozoa & helminths) • Inhabit body surfaces exposed to environment, such as : Skin- Oropharynx- Intestinal tract -Vagina. • Differ in number & kind at various anatomic sites. • Low-virulence organisms. • In their usual anatomic site, they are nonpathogenic. • If they leave their usual anatomic site, especially in immunocompromised individual disease. Batterjee Medical College Normal flora Colonization • Some people can be colonized: - transiently or for long periods - With certain organism (not members of normal flora). Colonization resistance It occurs when members of normal flora occupy receptor sites on skin & mucosal surfaces from binding to those receptors. preventing pathogens Batterjee Medical College Normal flora Batterjee Medical College Important Members of the Normal Flora Batterjee Medical College Bacterial Pathogenesis Batterjee Medical College Bacterial Pathogenesis Pathogen Pathogen are microbes capable of causing disease in immunocompetent people. Opportunistic Pathogen It refers to microbes that are capable of causing disease only in immunocompromised people. Carriers (chronic carriers) Individuals in whom pathogenic organisms are present in significant numbers & are source of infection for others. Batterjee Medical College Bacterial Pathogenesis Virulence It is measure of microbe's ability to cause disease Asymptomatic or inapparent Infections These infections occur when our host defenses have eliminated microorganism before it could multiply to sufficient numbers to cause symptoms of disease. Batterjee Medical College Bacterial Pathogenesis Infection Infection has two meanings: (1) Presence of microbes in body (2) Symptoms of disease. Presence of microbes in body does not always result in symptoms of disease Bacteria cause symptoms of disease by two main mechanisms: 1. Production of toxins (exotoxins & endotoxins) 2. Induction of inflammation. Batterjee Medical College Bacterial Pathogenesis Communicable Infection • Most bacterial infections are communicable, i.e., capable of spreading from person to person Epidemiologic terms used to describe infections Endemic :Infections that occur at persistent, usually low level in certain geographic area Epidemics: Infections occur at much higher rate than usual. Pandemics: Infection spread rapidly over large areas of globe Batterjee Medical College Bacterial Pathogenesis Stages of Bacterial Pathogenesis 1. Transmission from external source into portal of entry. 2. Evasion of primary host defenses (skin or stomach acid). 3. Adherence to mucous membranes, usually by bacterial pili. 4. Colonization by growth of bacteria at site of adherence. 5. Disease symptoms caused by toxin production or invasion accompanied by inflammation. 6. Host responses, both nonspecific & specific (immunity) 7. Progression or resolution of disease. Batterjee Medical College Determinants of Bacterial Pathogenesis: Transmission Mode of Transmission Comment I. Human to human A. Direct contact Intimate contact, e.g. •Sexual •Passage through birth canal B. No direct contact Fecal–oral, e.g., excreted in human feces, then ingested in food or water C. Transplacental Bacteria cross the placenta and infect the fetus D. Blood-borne • Transfused blood or intravenous drug use can transmit bacteria and viruses Batterjee Medical College Determinants of Bacterial Pathogenesis: Transmission Mode of Transmission Comment II. Nonhuman to human A. Soil source wound in skin B. Water source water aerosol are inhaled into lungs C. Animal source 1. Directly cat scratch 2. Via insect vector tick bite 3. Via animal excreta cattle feces are ingested in undercooked hamburger D. Fomite source towel, are transferred onto the skin Batterjee Medical College Determinants of Bacterial Pathogenesis: Transmission Vertical Transmission • It is transmission of Bacteria, viruses & other microbes from mother to offspring through: - Placenta - Birth canal during birth - Breast milk. Horizontal Transmission It is person-to-person transmission (not from mother to offspring). Zoonoses Human diseases for which animals are reservoir Batterjee Medical College Determinants of Bacterial Pathogenesis: Transmission The main "portals of entry" into body : • Respiratory tract • Gastrointestinal tract • Skin • Genital tract. Batterjee Medical College Determinants of Bacterial Pathogenesis: Toxin Production Property Source Exotoxin Endotoxin Certain species of gram-positive Cell wall of gram-negative and gram-negative bacteria bacteria Secreted from cell Yes No Chemistry Polypeptide Lipopolysaccharide Location of genes Plasmid or bacteriophage Bacterial chromosome Toxicity High (fatal dose on order of 1 g) Low (fatal dose on order of hundreds of micrograms) Clinical effects Various effects Fever, shock Mode of action Various modes Includes TNF and interleukin-1 Antigenicity Induces high-titer antibodies Poorly antigenic called antitoxins Vaccines Toxoids used as vaccines No toxoids formed and no vaccine Heat stability Destroyed rapidly at 60°C Stable at 100°C for 1 hour (except staphylococcal enterotoxin) Typical diseases Tetanus, botulism, diphtheria Meningococcemia, sepsis by gram-negative rods Batterjee Medical College Determinants of Bacterial Pathogenesis: Toxin Production Exotoxins • Many exotoxins have A–B subunit structure: - A (active) subunit possesses toxic activity - B (binding) subunit is responsible for binding exotoxin to specific receptors on membrane of human cell. Batterjee Medical College Typical Stages of Infectious Disease 1. Incubation period: time between moment person is exposed to microbe (or toxin) & appearance of symptoms. 2. Prodrome period: time during which nonspecific symptoms occur. 3. Specific-illness period: time during which characteristic features of disease occur. 4. Recovery period: time during which symptoms resolve & health is restored. Batterjee Medical College Typical Stages of Infectious Disease • After recovery period, some people become: - chronic carriers of organism - latent infections develop. • Some people have subclinical infections during which they remain asymptomatic.