Infectious diseases caused by bacterial agents are common and widespread. Because bacterial cells carry out the main functions of life they can be attacked by means of antibiotics which are chemical agents which interfere with the basic activities of the bacterial biochemistry. There are various antibiotics which interfere with specific enzyme pathways which are not found in normal human cells, for example many bacteria have a cell wall structure composed of hemicellulose and the manufacture of this material can be restricted by the use of penicillin and its derivatives.

The bacterial infection causes disease symptoms in two main ways, one is invasion of tissues and destruction of cells, and the other is the release of exotoxins and endotoxins. Exotoxins are released into the tissue fluids as waste products and they interfere with our biochemical systems usually by inhibition of key enzymes. Endotoxins are released by dying bacterial and human cells. There is evidence from plant infections that bacteria can carry plasmids which are introduced into the host cells and these can bind to the host’s chromosomes and cause them to alter their normal biochemical pathways, a similar process may well occur in some human bacterial infections.

The degree of damage a bacterial infection causes will depend upon a number of factors:

1. The invasiveness of the organism;
2. The type and effectiveness of toxins released;
3. The genetic make-up of the host human;
4. Dormancy which is the ability of the bacterium to enter an inactive state;
5. Infectivity - the dose rate needed to cause an infection.

Invasiveness: If the bacterium is able to enter tissues and spread freely through them then they are likely to do a great deal of damage to our system if their spread is not checked. Syphilis and typhoid fever are good examples of invasive bacteria. Antibiotics are very useful in treatment of these invasive types.

Toxins: Many bacteria cause death or injury due to the release of minute quantities of enzyme inhibiting waste products. The Tetanus bacterium is a good example it causes minor damage at its site of entry through a puncture wound but releases a toxin which inhibits the breakdown of acetylcholine at the post-synaptic nerve membrane. This results in failure of the skeletal muscles to relax and will finally cause death from asphyxia when the diaphragm and intercostal muscle fail to ventilate the lungs. Diphtheria is another good example, it causes a minor infection of the tonsil area, but releases toxins which rapidly cause death. Treatment for these diseases is usually best done preventatively by immunisation with modified toxin vaccine which adapts our system to manufacture the antitoxins required to defend the body before we are infected by the bacterium. The use of antitoxin (passive immunity) injections is an alternative if the person is infected before they can be actively immunised.

Genetic constitution: It is known that resistance to infection is related to the genotype of the individual. Certain individuals may carry antigens on their own cells which are so similar to those in the bacterium that they are unable to manufacture antibodies which are effective. These individuals would be more susceptible to the bacterial attack. Over millions of years we have been selected for resistance to those organisms which are common in our environment.

Dormancy: Some bacterial infections are characterised by some members of the population becoming dormant after the initial invasion. Antibiotics do not work against dormant individuals since they are not in a functional state their function cannot easily be disrupted! Tuberculosis (TB) is an example of this problem the treatment with antibiotics needs to be extremely long term so that the dormant individuals can be killed as they emerge from their dormant state. Short term treatment removes the active population but the dormant individuals will cause a resurgence of the disease symptoms at a later time.

Infectivity: The number of organisms required to cause the disease to develop into a symptomatic stage will vary greatly amongst different bacterial species and even within the strains of a single species. The Salmonella bacterium has a large number of strains, some produce dangerous toxins which result in food poisoning but a relatively large number must enter the gut with the food before they have an effect. However another strain causes typhoid fever and these need only to enter in small numbers, they do not produce toxins to any great extent but they invade the body tissues by passing through the gut wall and cause widespread damage to the body often resulting in death.

Some individuals act as carriers for the typhoid bacterium, they are infected with it but do not show any symptoms of disease, presumably their genetic make-up is different from your average human. these carriers can then pass on the bacterium by handling food which is eaten by others. There are strict rules of employment for anyone acting as a typhoid carrier in this country, but these rules do not apply abroad.

Some common Bacterial diseases

Prevention of Salmonella transmission

Salmonella bacteria occur in the guts of almost all species of animal, from cockroaches through terrapins to humans. Many strains are harmless but some are virulent. The pathogenic or virulent forms are able to attack our systems if they enter our gut and develop. They will enter with contaminated food or water and are especially common in poultry, which is why it should always be thoroughly cooked to kill the bacteria. The storage of cold cooked meats with uncooked meats is prohibited in the meat retailing industry of this country but not in your refrigerator, and this is a common source of infection, as is poor personal hygiene. If I pick up an uncooked chicken to put it into the fridge and then handle some cooked meat in the fridge to make a sandwich then I have infected the cooked meat. If I leave the sandwich aside for a couple of hours while I type these notes on the computer then rush down to eat the sandwich then I have given the organisms enough time to increase their numbers to the point at which they will cause food poisoning symptoms after the sandwich is ravenously devoured! And all this because I didn’t wash my hands after handling the chicken and before I grabbed the cold meat for the sandwich. Commonly, large scale outbreaks of food poisoning come about as the result of cooking poultry in a frozen condition. The inside of the meat is uncooked while the outside is all crispy. The meat is then sliced and placed on plates and left before the meal is served as a cold Salmonella buffet.

 Acquisition of virulence by bacteria.

Bacteria can mutate and give rise to new strains which have virulence genes. These new strains have characteristics of invasiveness or ability to produce toxins of a type which we cannot deal with. These bacteria will then have the capability to produce disease symptoms.

 Bacteria can be transformed. This is a technical term for a process much used in genetic engineering but which occurs as a natural part of the life of a bacterium. Some strains of bacteria can absorb DNA strands from their environment and if these carry virulence genes they will develop into a virulent strain. The DNA does not have to come from the same species of bacterium either!

Bacteria also have a means of passing pieces of DNA from one strain to another by a process called conjugation.

 

A bacterium containing a plasmid which has the genes necessary for conjugation is referred to as a positive strain (F⁺), it can form a tube called a pilus which will connect to another bacterial cell which is of a different strain (F^-). Once the pilus connects the two bacterial cells the double DNA molecule forming the plasmid separates into its two strands and one of these passes through the pilus and enters the other bacterium. The Plasmid DNA forms a circular coil and then each coil will copy its missing strand to form double stranded DNA molecules again. Now the F^-bacterium is transformed into an F⁺bacterium. Clearly the processes of transformation and conjugation can alter the genetic make-up of a bacterial strain and may cause the spread of virulence genes or may transfer antibiotic resistance genes from strain to strain or even between different species. This can speed up the selection of virulence or resistance in bacterial populations. The spread of antibiotic resistance is rapidly reducing the effectiveness of antibiotic treatments and investigations into the possibility of using bacteriophage viruses to control dangerous species of bacteria is underway.