Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Bacteria and bacterial infection

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otic can provide the bacteria with enough time to undergo

explosive increases in number, whereas the use of an antibiotic

to which the bacteria are susceptible can quickly quell a brew-

ing infection.

As with many other infections, bacteremic infections

can be prevented by observance of proper hygienic procedures

including hand washing, cleaning of wounds, and cleaning

sites of injections to temporarily free the surface of living bac-

teria. The rate of bacteremic infections due to surgery is much

less now than in the past, due to the advent of sterile surgical

procedures, but is still a serious concern.

See alsoBacteria and bacterial infection; Infection and

resistance

BBacteria and bacterial infectionACTERIA AND BACTERIAL INFECTION

Infectious diseases depend on the interplay between the abil-

ity of pathogens to invade and/or proliferate in the body and

the degree to which the body is able to resist. If the ability of

a microorganism to invade, proliferate, and cause damage in

the body exceeds the body’s protective capacities, a disease

state occurs. Infection refers to the growth of microorganisms

in the body of a host. Infection is not synonymous with disease

because infection does not always lead to injury, even if the

pathogen is potentially virulent (able to cause disease). In a

disease state, the host is harmed in some way, whereas infec-

tion refers to any situation in which a microorganism is estab-

lished and growing in a host, whether or not the host is

harmed.

The steps of pathogenesis, the progression of a disease

state, include entry, colonization, and growth. Pathogens like

bacteria use several strategies to establish virulence. The bac-

teria must usually gain access to host tissues and multiply

before damage can be done. In most cases this requires the

penetration of the skin, mucous membranes, or intestinal

epithelium, surfaces that normally act as microbial barriers.

Passage through the skin into subcutaneous layers almost

always occurs through wounds and in rare instances pathogens

penetrate through unbroken skin.

Most infections begin with the adherence of bacteria to

specific cells on the mucous membranes of the respiratory, ali-

mentary, or genitourinary tract. Many bacteria possess surface

macromolecules that bind to complementary acceptor mole-

cules on the surfaces of certain animal cells, thus promoting

specific and firm adherence. Certain of these macromolecules

are polysaccharides and form a meshwork of fibers called the

glycocalyx. This can be important for fixing bacteria to host

cells. Other proteins are specific, e.g., M-proteins on the sur-

face of Streptococcus pyogeneswhich facilitate binding to the

respiratory mucosal receptor. Also structures known as fim-

brae may be important in the attachment process. For exam-

ple, the fimbrae of Neiseria gonorrhoeaeplay a key role in the

attachment of this organism to the urogenital epithelium where

it causes a sexually transmitted disease. Also, it has been

shown that fimbriated strains of Escherichia coliare much

more frequent causes of urinary tract infections than strains

lacking fimbrae, showing that these structures can indeed pro-

mote the capacity of bacteria to cause infection.

The next stage of infection is invasion that is the pene-

tration of the epithelium to generate pathogenicity. At the

point of entry, usually at small breaks or lesions in the skin or

mucosal surfaces, growth is often established in the submu-

cosa. Growth can also be established on intact mucosal sur-

faces, especially if the normal flora is altered or eliminated.

Pathogen growth may also be established at sites distant from

the original point of entry. Access to distant, usually interior,

sites occurs through the blood or lymphatic system.

If a pathogen gains access to tissues by adhesion and

invasion, it must then multiply, a process called colonization.

Colonization requires that the pathogen bind to specific tissue

surface receptors and overcome any non-specific or immune

host defenses. The initial inoculum is rarely sufficient to cause

damage. A pathogen must grow within host tissues in order to

produce disease. If a pathogen is to grow, it must find appro-

priate nutrients and environmental conditions in the host.

Temperature, pHand reduction potential are environmental

factors that affect pathogen growth, but the availability of

microbial nutrients in host tissues is most important. Not all

nutrients may be plentiful in different regions. Soluble nutri-

ents such as sugars, amino acids and organic acids may often

be in short supply and organisms able to utilize complex nutri-

ent sources such as glycogen may be favored. Trace elements

may also be in short supply and can influence the establish-

ment of a pathogen. For example, iron is thought to have a

strong influence on microbial growth. Specific iron binding

proteins called transferrin and lactoferrin exist in human cells

and transfer iron through the body. Such is the affinity of these

proteins for iron, that microbial iron deficiency may be com-

mon and administration of a soluble iron salt may greatly

increase the virulence of some pathogens. Many bacteria pro-

duce iron-chelating compounds known as siderophores, which

help them to obtain iron from the environment. Some iron

chelators isolated from pathogenic bacteria are so efficient that

they can actually remove iron from host iron binding proteins.

For example, a siderophore called aerobactin, produced by

certain strains of E. coliand encoded by the Col V plasmid,

readily removes iron bound to transferring.

After initial entry, the organism often remains localized

and multiplies, producing a small focus of infection such as a

boil, carbuncle or pimple. For example, these commonly arise

from Staphylococcus infections of the skin. Alternatively, the

organism may pass through the lymphatic vessels and be

deposited in lymph nodes. If an organism reaches the blood, it

will be distributed to distal parts of the body, usually concen-

trating in the liver or spleen. Spread of the pathogen through

the blood and lymph systems can result in generalized (sys-

temic) infection of the body, with the organism growing in a

variety of tissues. If extensive bacterial growthin tissues

occurs, some of the organisms may be shed into the blood-

stream, a condition known as bacteremia.

A number of bacteria produce extracellular proteins,

which break down host tissues, encourage the spread of the

organism and aid the establishment and maintenance of dis-

ease. These proteins, which are mostly enzymes, are called

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