Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Infection and resistance

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large enough numbers, can germinate and cause severe illness

and even death.

To establish an infection, microbes must defeat two lines

of defense of the body. The first line of defense is at body sur-

faces that act as a barrier guard the boundaries between the body

and the outside world. These barriers include the skin, mucous

membranes in the nose and throat, and tiny hairs in the nose that

act to physically block invading organisms. Organisms can be

washed away from body surfaces by tears, bleeding, and sweat-

ing. These are non-specific mechanisms of resistance.

The body’s second line of defense involves the specific

mechanisms of the immune system, a coordinated response

involving a variety of cells and protein antibodies, whereby an

invading microorganism is recognized and destroyed. The

immune system can be strengthened by vaccination, which

supplies or stimulates the creation of antibodies to an organ-

ism that the body has not yet encountered.

An increasing cause of bacterial infectionis the ability

of the bacteria to resist the killing action of antibiotics. Within

the past decade, the problem of antibiotic resistant bacteria has

become a significant clinical issue. Part of the reason for the

development of resistance has been the widespread and some-

times inappropriate use of antibiotics (e.g., use of antibiotics

for viral illness because antibiotics are not effective against

viruses).

Resistance can have molecular origins. The mem-

brane(s) of the bacteria may become altered to make entry of

the antibacterial compound more difficult. Also, enzymescan

be made that will destroy or inactivate the antibacterial agent.

These resistance mechanisms can be passed on to subsequent

generations of bacteria that will then be able to survive in

increasing numbers.

Bacteria can also acquire resistance to antibiotics and

other antibacterial agents, even components of the immune

system, by growing on body surfaces, passages, and tissues. In

this mode of growth, termed a biofilm, the bacteria are

enmeshed in a sticky polymer produced by the cells. The poly-

mer and the slow, almost dormant, growth rate of the bacteria

protect them from antibacterial compounds that would other-

wise kill them, and can encourage the bacteria to become

A group of people with leprosy in the Middle East.

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