- Microbiology
As we start to look as Microbial-caused diseases, we should remember what we learned during the first section of Bio 113. The principle that molecular Interaction is based on molecular structure.

Biological molecules interact by recognizing and binding with one another in a highly specific manner dependent on their structure (shape). Pairs of molecules that interact in this way are called receptors and ligands respectively. Following this binding some interaction or event occurs, depending on the type of cells and system involved.

E Coli Attachment movie (HHMI)

Most microbes produce disease by first recognizing their specific hosts through molecular interaction and by then producing substances (e.g. toxins) that interact specifically with the host to cause disease. This is one reason for diseases that only affect certain organisms, like swine rhinitis, a bacterial disease that only Pigs can get.

In spite of our considerable knowledge about diseases and their causes, many people in the world still die of the same infectious diseases that killed our ancestors 10,000 years ago. The WHO estimates that approximately 15 million children die each year of preventable infectious diseases.

Unfortunately, the emergence of drug resistance in bacteria, parasites, viruses, and fungi is swiftly reversing the progress prompted by the many miracle drugs developed over the last 50 years. For example, in some areas of the United States, up to 30% of infections with Streptococcus pneumoniae, the most common cause of bacterial pneumonia, meningitis, and ear infections, are no longer susceptible to penicillin.

Every infectious disease is characterized by certain symptoms produced in the average victim of that disease. These symptoms, referred to as the Clinical symptoms, are used by physicians and health care personnel to identify a particular disease or group of diseases. The first step in treating an infectious disease is to describe the symptoms, then identify the agent (bacteria or virus) that caused the disease, and then determine the manner in which the infectious disease was contracted. Only after the identity of the infectious agent is determined, can appropriate treatment begin.

The diagnosis of a bacterial infection depends not just on the identification of the bacteria, but in establishing a plausible link between the bacteriological findings, recognized diseases and the patient's clinical condition. The capacity of a bacterium to cause disease reflects its relative pathogenicity. The pathogenicity of the bacteria and the susceptibility of the host , together determine the extent of the disease. Both of these factors can change with time, bacteria and viruses can become more pathogenic, and we can become more susceptible. Emerging infectious diseases are diseases of infectious origin whose incidence in humans has increased within the past two decades or that threaten to increase in the near future.

In the late 1800's a set of particular steps were developed that were used to identify infectious agents, in general those same steps are still followed today. They are called Koch's postulates (after the guy who developed them).

Koch's postulates

1. Isolate the suspected agent from a disease victim. 2. Grow the agent in pure culture. 3. Re-infect a healthy host with the agent and show that produces the same disease. 4., Re-isolate the same organism from the new victim.

There are many different types of bacteria