Chapter 13 – Microbe-Human Interactions

Acquisition of resident flora The human body supports a wide range of habitats Wide range of microbes can inhabit Resident flora or microflora Microbes that inhabit but do not harm the host

Locations on the host that harbor normal flora. Table 13.1 Sites that harbor a normal flora

Invasion of normally sterile regions of the body can result in infection and disease.

Factors that predispose a person to infections.

Exotoxins are released by the bacterium and directly affect different host organs, while endotoxins are released after the bacterium is lysed.

Toxins Toxigenicity – ability to make toxins Toxemia – toxin spread from site of infection by blood Intoxication – ingestion of toxins (botulism) Neurotoxins act on nervous system Enterotoxins act on intestine Hemotoxins lyse red blood cells Nephrotoxins damage kidneys

Process of Disease Localized infection – warts Systemic infection – spreads to several sites; measles, AIDS Focal infection – local infection which can spread, causing local infections in other parts of the body. tuberculosis, strep throat Primary – initial infection Secondary – due to altered defenses Acute vs. Chronic

Signs and symptoms Signs - objective evidence of disease based on observation Lesions, inflammation, pus Symptoms – subjective evidence of disease based on the patient Fatigue, dizziness, nausea Syndrome – a specific group of signs & symptoms that accompany a particular disease.

Epidemiology Prevalence of disease – number of cases with respect to entire population: usually reported as a % Incidence – number of new cases over a certain amount of time: usually reported as X per 100,000 people per month, week, year etc.

The frequency of a disease in a population can be used to defined endemic, epidemic, sporadic, and pandemic diseases.

Reservoirs of Infection A continual source of the pathogen, such an animal or inanimate object is a reservoir of infection Living Sick individuals Carriers – infected but asymptomatic Passive carriers – health care workers Vectors – live animals that transmit infectious agents from host to host

Vectors Biological Participates in the pathogen’s life cycle Infected with the pathogen Transmit by bites, defecation Mechanical Not part of pathogen’s life cycle Not infected with the pathogen

A mosquito is a biological vector and the common house fly is a mechanical vector. Fig. 13.13 Two types of vectors.

Reservoirs of Infection Nonliving Soil Transmit bacteria, protozoa, helminths, fungi Transmit spores, cysts, ova, larvae Water – similar to soil

Acquisition and Transmission Communicable Non-communicable Patterns of transmission

Communicable An infected host can transmit the infectious agent to another host and establish infection in that host.

Non-communicable Does NOT arise through transmission of the infectious agent from host to host. Host acquires infectious agent From self (compromised individuals can be infected by their own microflora) Nonliving reservoir – soil e.g., tetanus

Patterns of transmission Horizontal Vertical Direct (contact) Indirect

Horizontal Disease is spread through a population from one infected person to another Kissing, sneezing

Vertical The disease is transmitted from parent to offspring Ovum, sperm, placenta, milk

Direct (contact) Some type of contact must occur between skin or mucous membranes of the infected person and that of the new contact Kissing, sex Droplets Vertical Vector

Indirect Contaminated materials Air Food, water, biological products (blood, serum, tissue), fomites (door knobs, toilet seats, etc.) Oral-fecal Air Droplet nuclei (dried microscopic residue) Aerosols (dust or moisture particles)

Communicable diseases are acquired by contact and indirect transfer.

A sneeze can release enormous amounts of moist droplets, and the dry droplets form droplet nuclei.

Nosocomial Infections Infectious diseases that are acquired or developed from a hospital stay Urinary tract infections Respiratory infections Surgical incisions

The most common nosocomial infections.

Koch’s postulates Method used to determine the etiologic agent Ex. Toxic shock syndrome, AIDS, Lyme disease, Legionnaires

Steps associated with Koch’s postulates. Fig. 13.17 Koch’s postulates: Is this the etiologic agent?