Infectious Agents - Overview

Introduction

Infectious agents are microorganisms found in nature that are harmful to animals and plants. Most microorganisms found in nature are not harmful, however some microorganisms can cause disease and are therefore referred to as pathogenic.

Microorganisms are categorised into distinct types. The main categories of veterinary interest are bacteria, fungi, protozoa, viruses and prions, although other microorganisms also exist such as algae.

Bacteria

Bacteria are unicellular prokaryotes, usually with rigid cell walls. They are much smaller than eukaryotic cells, although they can easily be viewed with traditional light microscopy.
They have various shapes or morphologies such as rods, cocci, helical and branching and motile bacteria possess flagella. Bacteria contain a single circular chromosome containing DNA and reproduce by a process called binary fission which is an asexual process. Some bacteria also contain small extra-chromosomal DNA, referred to as plasmids, which have genes that are able to confer specific properties to bacteria such as antibiotic resistance.

There are two main groups (or domains) of bacteria; Archaebacteria and Eubacteria. Only Eubacteria are pathogenic in domestic species.

Fungi

Fungi are non-photosynthesising eukaryotes including yeasts, moulds and mushrooms which are either unicellular or multi-cellular. Moulds and mushrooms are multi-cellular and produce filamentous microscopic structures whilst yeasts are unicellular and have a spherical or ovoid shape. Unicellular fungi multiply by budding.

Cells found in moulds are cylindrical and attached end to end forming branched structures called hyphae.

Some types of fungi are classed as pathogenic due to their ability to secrete mycotoxins that are able to cause disease.

Viruses

The most notable difference between bacteria, fungi and viruses is that viruses are not cells. Viruses consist of nucleic acid, either DNA or RNA which is enclosed in a protective protein coat called a capsid. Viruses have various shapes ranging from spherical to elongated.

As viruses are not cells, they do not possess the ability to undertake metabolism and independant reproduction. Therefore viruses can only multiply within living cells by making use of the cells own structures and enzymes. Prokaryotic and eukaryotic cells are susceptible to infection by viruses.
Viruses are very small and not visible under light microscopy. Scanning electron microscopes using a magnification of 100,000x are used to demonstrate viruses in clinical specimens.