Introduction

Bacteriology is the branch and specialty of biology that studies the morphology, ecology, genetics and biochemistry of bacteria as well as many other aspects related to them. This subdivision of microbiology involves the identification, classification, and characterization of bacterial species. Because of the similarity of thinking and working with microorganisms other than bacteria, such as fungi, and viruses, there has been a tendency for the field of bacteriology to extend as microbiology.

Bacteria are single celled microbes. The cell structure is simpler than that of other organisms as there is no nucleus or membrane bound organelles. Instead their control centre containing the genetic information is contained in a single loop of DNA. Some bacteria have an extra circle of genetic material called a plasmid. The plasmid often contains genes that give the bacterium some advantage over other bacteria. For example it may contain a gene that makes the bacterium resistant to a certain antibiotic.

Bacteria are classified into 5 groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios) or corkscrew (spirochaetes). They can exist as single cells, in pairs, chains or clusters.

Classification of Bacteria

Types of Bacteria

There are seven main groups of bacteria, distinguished by their shape and the type of cell wall they possess. Four of the seven types make up the majority of all bacteria:
Gram positive cocci
Gram negative cocci
Gram positive bacilli
Gram negative bacilli
Cocci are spherical cells, bacilli are rod-shaped. Bacteria of either shape that have thick cell walls are termed gram positive because of the way they take up the Gram stain. Those with thin cell walls are termed gram negative. These four main types are described in more detail below.
There are also three other, more rare types of bacteria:
Spirochaetes: corkscrew-shaped bacteria that fall into three species, all of which cause specific diseases. Treponema bacteria cause syphilis and yaws, Leptospira bacteria cause leptospirosis and Borrelia bacteria cause Lyme disease.
Rickettsia: bacteria from the Rickettsiaceae family, which can only live and survive inside other living cells. One type causes the disease typhus.
Mycoplasma: bacteria that have no cell wall. These can infect humans and cause pneumonia.

Gram Positive Bacteria

The gram positive cocci include the well known species Staphylococcus and Streptococcus. Bacteria from both species fall into the category of friendly bacteria; they do no harm and perform useful functions in the human body and in the environment. Some species can also be pathogenic. Staphylococcus aureus can cause impetigo and scalded skin syndrome, food poisoning and toxic shock syndrome. Streptococcus pyogenes is the culprit usually responsible for tonsillitis and severe sore throats (‘strep throat’), but it can cause many other infections.
Gram positive bacilli include Corynebacterium diphtheriae, which causes diphtheria, Listeria monocytogenes, found in unpasteurized dairy products and responsible for dangerous infectious in pregnant women, and bacteria from the species Lactobacillus, friendly bacteria found in the gut.

This group also includes two of the most dangerous types of bacteria known to humans. One is the Bacillus species that causes anthrax, and the other is Clostridium. One Clostridium species causes tetanus, another leads to botulism, a deadly form of food poisoning.

Gram Negative Bacteria

There are two main types of gram negative cocci, both belonging to the genus Neisseria. Neisseria meningitidis causes a form of meningitis, Neisseria gonorrhoeae causes the sexually transmitted infection gonorrhae. The two species are more commonly called the meningococcus and the gonococcus.
Gram negative bacilli are a large and varied group that are subdivided into several further categories. The Enterobacteria include many species that cause food poisoning in humans – E. coli, Salmonella, Shigella, Proteus, and also the plague bacterium Yersinia pestis. The Vibrio group contain bacteria that are shaped more like commas than rods – and include the bug that is responsible for cholera.

Helicobacter pylori (H. pylori) is also a gram negative bacillus. This bacterium has been identified in the last 25 years as a major cause of stomach ulcers.

Other gram negative bacilli are Hemophilia influenzae, which causes pneumonia, Bordetella pertussis, which causes whooping cough, and Brucella bacteria, which are associated with brucellosis in cattle. A final group is the Bacteroides, a species of bacteria that are very common in the human gut. In fact, they make up a quarter of the dead bacteria in faeces.