MicrobiologyBytes: Infection & Immunity: Man & microbes Updated: October 21, 2004 Search

Man and Microbes

Aims and Objectives

  1. To know the different types of micro-organisms.
  2. To understand their structural differences.
  3. To understand that not all micro-organisms produce disease.

Introduction

Man is only one species in a world inhabited by many different flora and fauna. Like all other living organisms we inhabit an ecological niche in which we survive. Micro-organisms could be considered to be the greatest threat to our survival. In this course we will be looking at the balance which is maintained between those micro-organisms which cause infectious disease and ourselves. In this document we consider the important micro-organisms, the 'away team', and our host defences, 'the home team'.

THE AWAY TEAM

Micro-organisms are organisms which share the property of being sub-microscopic. They existed long before man and we have had to adapt to live in an environment where they are far more abundant. Many micro-organisms do not normally cause disease in man, existing in a state of either COMMENSALISM, where there is little or no benefit or harm to man, or in MUTUALISM, where there is some benefit gained by both partners. This non-harmful balance exists when the immune system works well, but these same organisms can cause infection when the latter fails. This changed property is shared by many other micro-organisms which can cause infectious disease in the immunocompetent individual, so-called PATHOGENS. These pathogens fall into five main groups:

In addition, by convention there are two further classes of small agents which are often included in the study of MICROBIOLOGY:

As these latter organisms cause infestations rather than infectious disease they will not be further considered in this course.

Basic differences between different types of micro-organisms

The basic distinguishing features of the major classes of micro-organisms are in the following table.

VIRUSES
BACTERIA
FUNGI
PROTOZOA
HELMINTHS
Cell structure
Not applicable
Prokaryotic
Eukaryotic
Eukaryotic
Eukaryotic
Size
+
++
+++
++++
+++++
DNA and RNA
No
Yes
Yes
Yes
Yes
Living?
No
Yes
Yes
Yes
Yes
Nucleus?
Not applicable
No
Yes
Yes
Yes
Ribosomes
Not applicable
70S
80S
80S
80S
Internal organelles
Not applicable
No
Yes
Yes
Yes
Multicellular?
Not applicable
No
Both
No
Yes
Cell wall
Not applicable
Yes, peptidoglycan
Yes, chitin
No
No

Prokaryotic cells do not have a nucleus, have a cell wall, possess 70S ribosomes, no internal organelles, no cytoskeleton and have a non-"9+2" flagellar arrangement. Eukaryotic cells possess a nucleus, have 80S ribosomes, have internal organelles and a cytoskeleton, possess flagellae with a 9+2 arrangement if present, and may lack a cell wall.

Microbes and human disease
The normal result of an interaction between a micro-organism that is not a commensal and man is INFECTION. Infection is the invasion or colonisation of the body by pathogenic organisms. DISEASE results when this results in a change of the state of health. The manner by which the disease develops is called PATHOGENESIS.

 

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Infectious disease is by far the greatest cause of morbidity and mortality world-wide. Indeed respiratory infections and gastrointestinal infections cause more deaths world-wide than all other diseases added together. The consequences of infectious disease are protean. They range from the common cold to AIDS. No system of the body escapes infection. In addition there are some general features of infectious diseases:

© AJC 2007.