How Bacteria Infect the Body and Cause Disease

Bacterial disease always starts with infection, when bacteria enter the body, bypassing the first-line physical and chemical defenses of the immune system. Skin and mucous membranes are physical barriers to infectious agents. These physical barriers also produce chemicals such as perspiration, sebum and enzymes that can destroy potential pathogens. Yet microbes are sometimes still able to breach the system.

What Is a Bacterial Infection?

When bacteria enter the body they "infect". But this term applies to the presence of bacteria that may or may not result in illness. In fact, many types of bacteria normally exist in the body. These bacteria are called "normal flora", and have a mutually beneficial, symbiotic relationship with their host. Some create vitamins, other help us digest food, or just crowd out other species of bacteria that could harm the body.

Infection Versus Disease

Infection does not equate with disease. The term "infection" only means that an infectious agent, such as bacteria, has been introduced into the system.

Infection results when:

• pathogenic (bad guy) bacteria enter the body
• normally beneficial bacteria gets into an area of the body that is supposed to be sterile
• conditions of the body (such as pH) change, allowing certain microbes to overpopulate

The term disease, in relation to infectious agents, applies when the presence of bacteria damages the body and causes illness.

Bladder Infection: Normal Flora in the Wrong Place

For example, most bladder infections, a condition also known as cystitis, are caused by Gram-negative enteric (GI tract) bacteria that have made their way from the anus to the external genitalia and then up the urethra to the bladder. Enteric bacteria are necessary for digestive health, but they are only beneficial when in the digestive tract, where they belong. By contrast, the bladder is normally a sterile area of the body, so presence of bacteria is abnormal.

Enteric bacteria that enter the bladder are able to hang on to the mucous membrane where their grappled populations quickly grow to the point that uncomfortable symptoms result (burning, pain, urgent need to urinate). Women are at higher risk of bladder infection because of a short urethra and its close proximity to the anus. Sulfa antibiotics are typically prescribed to combat this type of infection.

Yeast Vaginitis Infection: Overpopulation of Normal Flora

After being treated for a bladder infection, many women end up developing another problem--vaginitis, a yeast infection. Yeast are not bacteria, but are another type of microbe, fungi, that has the potential to cause disease.

Yeast are normally present in the vagina in low numbers. Their population growth is slowed by the normally acidic environment of the vagina. The presence of bacteria helps to create this acidic environment, and the bacteria also compete with the yeast, limiting their growth.

Most antibiotics kill both disease-causing bacteria and normal bacterial flora. They are not precise. During a course of antibiotics to relieve a bladder infection, bacteria are also killed in other areas of the body, such as the vagina. The absence of bacteria in the vagina allows the yeast population to expand.

Yeast, like other fungi, obtain nutrition by secreting enzymes that break down surrounding organic material. When this happens on living tissue, it is irritating and uncomfortable, to say the least. To treat the yeast infection, an anti-fungal medication must then be prescribed.

To learn more about microbes and infectious disease, see the Virtual Microbiology Classroom.

Sources

Bauman, R. (2007) Microbiology with Diseases & Taxonomy, Pearson Benjamin Cummings.

MedicineNet (2010), "Vaginal Yeast Infection".

WebMD (2010) Understanding Bladder Infections: Basic Information.