Molecular Microbiology Information

Bacterial Virulence Factors : Supreantigens

Superantigens are bacterial proteins that bind to MHC class II on APCs and to T cell receptors to force an unnatural association between APCs and T cells.

This results in release of high levels of IL-2 and ultimately causes other cytokines to be released.

Bacterial Virulence Factors : Membrane-disruption toxins

Membrane-disruption toxins toxins disrupt host cell membranes either by forming channels in the membrane or hydrolyzing phospholipds in the membrane.

These toxins kill host cells by lysing them or also aiding phagocytosed bacteria to escape the phagocytic vesicle and enter the host cell cytoplasm.

Bacterial Virulence Factors : A-B exotoxins

A-B exotoxins contain one portion (B) that binds to the host cell and a enzymatically active second portion (A).

The portion B may be a single polypeptide or consist of several subunits. The host cell receptor recognized by the B poA-B exotoxins contain one portion (B) that binds to the host cell and a enzymatically active second portion (A).The portion B may be a single polypeptide or consist of several subunits. The host cell receptor recognized by the B portion is usually, but not always, a host cell surface carbohydrate.

The portion A of most A-B exotoxins inactivates a host protein by covalently attaching an ADP-ribosyl group to it (ADP-ribosylation).

Bacterial Virulence Factors

Exotoxins are toxic proteins produced by some gram-negative and gram positive bacteria. Many of these toxins are excreted into the medium but some toxins are released only during bacterial lysis.

Some exotoxins (cytotoxins) attack a variety of cell types while others (e.g. neurotoxins and hepatotoxin) attack a specific cell type.

There are three types of exotoxins:

• A-B toxins
• Membrane-disruption toxins
• Superantigens

Exotoxins plays a variety of roles in bacterial disease. Toxin formed in food and then ingested along with the food can be the sole source of symptoms.

Bacteria colonized a wound or mucosal surface but do not invade can produce toxins that act locally or enter the bloodstream and attack internal organs.

Bacteria growing in a wound can produce toxins that destroy host tissue and kill phagocytes in the immediate vicinity of the bacteria, thus facilitating bacterial growth and spread.

Design of vaccines is determined by where a toxin is produced and where it acts. Nontoxic forms of a toxin (toxoid) are used to elicit an antibody response that neutralizes the toxin.

Neisseria gonorrhoeae

Gonorrhea is caused by Neisseria gonorrhoeae, a gram-negative diplococcus which requires CO2 and selective media for growth.The bacteria are fragile and do not survive in the environment, so spread occurs only by direct contact (sexual contact).

N. gonorrhoeae adheres to the columnar epithelial cells of the cervix and urethra (genital gonorrhea) but can also adhere to the mucosa of the throat (oral gonorrhea) and rectum (rectal gonorrhea).

Colonization of the male urethra causes copious discharge and painful urination. In females the symptoms are purulent discharge, pain, and itching.

Colonization of the throat causes pharyngitis.

About 10% of men and nearly one third of women infected with N. gonorrhoeae are asymptomatic. Such asymptomatic carriers are a major factor in the continuing spread of the diseases.

Due to increase in antibiotics resistance, it has be imperative to develop an effective vaccine against N. gonorrhoeae.

Gonorrhea has several serious complication:

1. Women whose cervix is infected can transmit the disease to their infants during birth. Infection of the eyes of the infant can cause blindness. This can be prevented by routine use of antibacterial eye drops by hospitals.
2. Bacteria from the cervix can also ascend into the uterine cavity and infect the fallopian tubes, causing salpingitis or pelvic inflammatory response to the bacteria can lead to infertility and ectopic pregnancy.
3. In about 1% of infected men and women, the disease becomes systemic and can result in arthritis, endocarditis, and meningitis.