MICROBE STRUCTURE

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.

MICRO-ORGANISMS

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, the so-called PATHOGENS.

These pathogens fall into three main groups:

Viruses

Bacteria

Fungi

BASIC DIFFERENCES BETWEEN DIFFERENT TYPES OF MICRO-ORGANISMS

Bacteria

Structure of a typical bacteria

Bacteria are very small single celled organisms. They come in different shapes, but the most common are cocci (round) and bacilli (rod shaped). They have a cell wall around a cell membrane. There are no true cell organelles but the cell membrane is often folded in places to create primitive mitochondria or chloroplasts. There is no nucleus. The single circular chromosome is effectively 'naked' in the cytoplasm. There are often many smaller circular pieces of DNA in the cytoplasm known as plasmids. They often contain genes for antibiotic resistance. Bacteria can multiply very quickly under the right conditions, often doubling in number every 20 minutes.Pathogenic bacteria (those that cause disease) harm the body when they multiply rapidly, competing with the body's own cells for nutrients. The bacteria also often produce toxins, some of which are very poisonous and can kill.

Not all bacteria are harmful. Some are actually useful. The bacteria Lactobacillus is added to milk to make yoghurt. The bacteria feed on the lactose sugar found in milk and produce lactic acid. This turns the milk slightly sour which in turn clots the milk to produce natural yoghurt. Acetobacter is used to convert alcohol into vinegar. Some cheeses are ripened using enzymes from bacteria.

Viruses

Structure of a typical virus

These microbes are not 'alive' in the strictest sense. They consist basically of genetic material (DNA or RNA) surrounded by a protein coat. They can only reproduce which they gain entry to living cells. When inside a host, the genetic material inserts itself into the chromosomes of the host. The infected cell then uses these new genes to reconstruct many more virus particles. These can be released from the cell by budding at the cell membrane, or in more dramatic circumstances when the cell bursts open.

Fungi

Structure of a typical fungus

These microbes come in the form of single cells like yeast or thread-like structures like mould. As well as causing diseases like athletes foot, fungi can be useful. Yeast is perhaps the best known and has been used for hundreds of years in bread and wine making. The yeast is used in bread making to make the bread rise. As the yeast respire, they release carbon dioxide gas which gets trapped in the dough creating tiny pockets of gas. When baked they bubble expand to give bread its fluffy texture. Yeast is used in beer and wine making as when it respires anaerobically it ferments sugars into ethanol.

Information on Microbes and Disease

Microbiology - the Beginning