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II. MICROBIAL MECHANISMS OF PATHOGENICITY (CHAPTER 15) 1. INTRODUCTION a) Pathogenicity is the ability of a pathogen to produce a disease by overcoming the defenses of the host. b) Virulence is the degree of pathogenicity. 2. HOW MICROORGANIMS ENTER A HOST a) Pathogens gain access to the body by its portal of entry. A. PORTALS OF ENTRY (table 15.1 pg 439) i) Mucous membranes a) Many microbes can penetrate mucous membranes of the conjunctiva and the respiratory, gastrointestinal (GI) and genitourinary tracts. a. Microbes that are inhaled with droplets of moisture and dust particles gain access to the respiratory tract. b. Microbes enter the GI tract by ingestion via food, water, and contaminated fingers. Most of these care destroyed by HCl. ii) Skin a) Most microbes cannot penetrate intact skin, but may enter through openings such as hair follicles or sweat ducts. b) Some larvae burrow through the skin (hookworm). c) Some fungi infect the skin itself, growing on the keratin layer. iii) Parenteral route a) Some microbes can gain access to tissues by direct penetration (inoculation) through the skin and mucous membranes in bites, injections, and other wounds. B. THE PREFERRED PORTAL OF ENTRY a) Many microbes only cause infections when they enter the body through their own specific portal of entry. If they get in via another portal, they may not cause disease. C. NUMBERS OF INVADING MICROBES 1
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a) Microbes will usually not cause disease if they enter the body in small numbers. b) Virulence can be expressed ID 50 (infectious dose for 50% of the inoculated hosts). c) Potency of a toxin is expressed as LD 50 (lethal dose for 50% of the inoculated hosts). D. ADHERENCE (Figure 15.5 pg 441)
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This note was uploaded on 04/07/2008 for the course BIOLOGY BIO230 taught by Professor Prof.? during the Spring '08 term at Borough of Manhattan Community College.

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