MICROBIAL INFECTIONHow do microbes cause disease and how do we defend ourselves

from infection?

Professor: Dr. HUL Seingheng

Student Group 4: Miss. LOUN Botom Mr. LY Bunleat Mr. LY Chenghour Miss. MAI Sara Mr. LEANG Saingleng

Introduction

The vast majority of microbes that humans encounter on a dialy do no harm.

Only a minority of microbes can interact with humans to cause disease.

Some infections are chronic, developing slowly over many months or years, other may be rapily fatal.

Infections spread from a source known as the reservoir of infection . Soil is the reservoir of infection for tetanus(Clostridium tetani) and water for Legionnaire’s disease(Legionella pneumophila).

IntroductionIf the source of infection is the patient’s commensal flora,

the infection is said to be endogenous, but if it is acquired from elsewhere the infection is said to be exogenous.

When apparently unrelated cases of infextion occur within a population, they are said to be sporadic. If a disease is continuously present in a community it is endemic. Epidemics occur when the number of cases of a particular infection rise significantly above the endemic level. Pandemics are epidemics that cover the whole world.

Introduction Infections maybe transmitted in a number of ways.

These include from person-to-person, air-borne(Legionella pneumophila), water-borne, food-borne and insect-borne infection

1. Person-to-Person: Infections are spread by the:- Inhalation of infection droplets.- Respiration

Introduction- Sexual(venereal diseases)- Direct inoculation(acupuncture clinics and some

hospitals).Most infections that spread from one individual to another

are said to spread horizontally, through a population. When , a mother infects her baby the infection is said to be spread vertically(development of the fetus).

Introduction 2 . Insect-borne causes the Plague.

Some other it causes malaria, dengue and yellow fever.Some pathogens can cause disease by intoxication. In such

cases, live microbes do not need to enter the body(staphylococcal food poisoning or more serious). Other pathogens colonise the body surfaces(the viruses that cause the common cold and also Vibrio cholerae cause of cholera).

There are microbes that penetrate the body(Salmonella typhi) lives inside white blood cells that we use to protect ourselves from infection.

Animal modelsAnimal models have been used in an attempt to measure

the virulence of an organism. Although some animal models mimic human infection well(ferrets and influenza). But these studies have not always been satisfactory example:

- Like Salmonella typhi cause an enteric fever with human but it cause a mild gastrointestinal disease with mouse model.

- In contrast, Salmonella typhimurium cause human gastroenteritis but in the mouse it cause enteric fever (mouse typhoid).

The response to infection

The degree of infection to which individuals maybe respond highly variable. During 1926, children in Lubeck in Germany were accidentally vaccinated with a virulent strain of Mycobacterium tuberculosis rather than the attenuated vaccine strain. Of the 249 children given the virulent vaccine, 76 died. The remainder developed only minor lesions. Consequently, the defferent responses of the children to the vaccine reflect defferences in the host response to infection.

The response to infection

Another problem with the study of infection is in some cases a single pathogen is associated with a single disease process(Yersinia pastis causes plague) and other microbes cause a range of diseases(Staphylococcus aureus range trivial conditions like staphylococcal food poisoning, impetigo and boils).

The study of pathogenesis is fraught with problem. Robert Koch attempted to define that a particular microbe

cause a particular infection.

Control of infection

Our innate defences naturally protect us from infection. Sweat on the skin is an excretion with a very high the concentration of salt provides additional problems for microbes trying to colonise skin.

The commensal flora that manages to live on the skin can metabolise the fatty secretions in sebum to yield free fatty acids can be potently antimicrobial.

Skin is also a hard and multi-layer structure with flakes being sloughed off continually.

Control of infection

Phagocytic leukocytes are important in providing a cellular defence against infection.

Once inside the Phagocyte, the lysosomes within the cell discharge antimicrobial chemicals.

There are leukocytes known as natural killer(NK) cells that attack cells infected with viruses.

We are also protected by our complement system. This is a collection of proteins that act together to produce a cascade response.

Control of infection

The complement system has two major effects: it can act directly in invading microbes or it can act in association with antibody to cause cell lysis.

The complement system also binds to the outside of microbes, making them much easier for phagocytes to engulf.

Lymphoid cells consist of B and T lymphocytes. B lymphocytes protect us by secreting specific antibody molecules that interact with particular pathogens to destroy them. T lymphocytes interact directly with pathogen.

Control of infection

Antibodies are also known as immunoglobulins. There are IgM and IgG: Y-ishaped glycooproteins that can interact with specific sites on antigens known as epitopes. They comprise heavy and light chain.

The first antibody to appear in response to infection is IgM but it is not as effective as IgG. It is the antibody class that is important in maintaining our immunity over time since it interacts very tightly with its epitope.

Control of infection

Although protection from infection is conferred by antibody, cell-mediated immunity is often of primary importance in recovering from an infection. cell-mediated immunity is of particular importance in protecting form parasite infestation, may virus infections and in chronic diseases such as tuberculosis. Central to the cell-mediated immune response are the T lymphocytes, so called because they pass through and

Control of infection

All our defences operate to keep us healthy and free from infection. Nowhere is this apparent than when people have to spend time in hospitals.

About one hospital patient in every ten acquires and infection as a direct result of their stay in hospital. Such infection are reffered to as nosocomial infection.

These bacteria re rarely found to cause infection in the population at large but are often seen causing infection in hospitalised patients.

Frequently they are adapted to the hospital enviroment and are often resistant to a range of antibiotics.

Control of infection

Control of nosocomial infection is a full time job. Each hospital has an infection control team.

It is the role of the hospital infection control team to monitor the level of nosocomial infection and, when incidents arise, to respond by identifying the source of the infection and its mode of spread.

With this information, it is usually a simple matter to break the cycle of infection.

Control of infection

Face masks stop infection from air-borne droplets. Surgical gloves prevent imcrobes from the surgeon’s hands from entering wounds.

Sterilisation of surgicaal instruments and other medical items ensures that pathogens are not accidentally introduced into the patient underging medical procedures.

Patients who are suffering from highly contagious infections are often barrier nursed.

Anything that that comes into contact with the patient is considered contaminated and is sterilised before being returned into general use.

Control of infection

Personnel are protected by special clothing, gloves and masks.

The lessons learned from barrier-nursing techniques can also be used to protect patients who are most vulnerable to infection.

ConclusionFinally we can know how do microbes cause

diseases and how do we defend ourselves, how to response the infection and how to control of infection.

Thank for your attention