Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Antibiotic resistance, tests for

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Some strains of E. colithat infect intestinal cells do so

by manufacturing and then releasing an adhesin, which is

incorporated into the membrane of the host cell. Thus, the bac-

teria install their own receptor in the host tissue.

Adhesion need not rely on the presence of adhesins. The

chemistry of the surface can also drive adhesion. For example,

the surface of the spores of bacillus and the capsule surround-

ing Pasteurella multocidaare described as being hydrophobic;

that is, they tend not to associate with water. This hydropho-

bicity will drive the spore or bacterium to associate with a sur-

face of similar chemistry.

In order to block adhesion that is the result of the above

mechanisms, the molecular details of these mechanisms must

be unraveled. This is an on-going process, but advances are

being made through research.

Adhesion of Escherichia colican depend on the pres-

ence of an adhesin called FimH. Antibodies to FimH can block

adhesion, presumable by binding to the FimH protein, pre-

venting that protein from binding to the receptor on the surface

of the host cell. Furthermore, the three-dimensional structure

of this adhesion is similar to that of adhesins from other bac-

teria. Avaccinedevised against FimH might then have some

protective effect against the adhesion of other bacteria.

In the case of the capsule-mediated adhesion, such as

the example above, capsular antibodies may also thwart adhe-

sion. The drawback with this approach is that capsular mate-

rial is not a potent stimulator of the immune system.

For microorganisms that secrete their own receptor,

such as Escherichia coli, or which have receptor molecules

protruding from their own surface (an example is the hemag-

glutininprotein on the surface of Bordetella pertussis), adhe-

sion could be eliminated by blocking the manufacture or the

release of the receptor molecule.

In Canada, field trials began in the summer of 2001 on

a vaccine to the adhesin target of Escherichia coli O157:H7.

This pathogen, which can be permanently debilitating and

even lethal to humans who ingest contaminated food or water,

often lives in the intestinal tracts of cattle. By eliminating the

adhesion of the bacteria, they could be “flushed” out of the

cattle. Thus, a vital reservoir of infection would have been

overcome. The vaccine could be ready for the market by as

early as 2003.

Another anti-adhesion strategy is to out-compete the

target bacteria for the available spots on the surface. This

approach has been successful in preventing bacterial vaginal

infections. Suppositories loaded with bacteria called

Lactobacillusare administered. Colonization of the vaginal

wall by the Lactobacillus can retard or even prevent the sub-

sequent colonization of the wall by a harmful type of bacteria.

The same bacteria are present in yogurt. Indeed, consumption

of yogurt may help prevent intestinal upset due to colonization

of the gut by harmful organisms.

Non-living surfaces, such as catheters and other

implanted material, are colonized by, in particular, bacteria. In

seeking to prevent adhesion, scientists have been experiment-

ing with different implant materials, with the incorporation of

antimicrobial compounds into the implant material, and with

the “pre-coating” of the material. In the case of antimicrobial

compounds, promising results have been obtained in labora-

tory studies using material that can slowly release antibiotics.

The disadvantage of this approach is that the presence of resid-

ual antibiotic could encourage the formation of resistance.

Pre-coating implant material with an antimicrobial compound

that is permanently bonded has also been promising in lab

studies.

See alsoBiofilm formation and dynamic behavior; Infection

and resistance; Probiotics

AAntibiotic resistance, tests forNTIBIOTIC RESISTANCE, TESTS FOR

Bacteriacan sometimes adapt to the antibioticsused to kill

them. This adaptation, which can involve structural changes or

the production of enzymesthat render the antibiotic useless,

can make the particular bacterial species resistant to the par-

ticular antibiotic. Furthermore, a given bacterial species will

usually display a spectrum of susceptibilities to antibiotics,

with some antibiotics being very effective and others totally

ineffective. For another bacterial species, the pattern of antibi-

otic sensitivity and resistance will be different. Thus, for diag-

nosis of an infection and for clinical decisions regarding the

best treatment, tests of an organism’s response to antibiotics

are essential.

A standard method of testing for antibiotic resistance

involves growth of the target bacteria in the presence of vari-

ous concentrations of the antibiotic of interest. Typically, this

test is performed in a specially designed plastic dish that can

be filled with agar(a Petri plate). Contaminationof the agar,

which would spoil the test results, is guaranteed by the steril-

ity of the plate and the lid that fits over the agar-containing

dish. The type of agar used is essential for the validity of the

tests results. Typically, Iso-Sensitest agar is used.

The hardened agar surface receives a suspension of the

test bacteria, which is then spread out evenly over the surface

of the agar. The intention is to form a so-called lawn of organ-

isms as growth occurs. Also on the agar surface are discs of an

absorbent material. A plate is large enough to house six discs.

Each disc has been soaked in a known and different concen-

tration of the same or of different antibiotics.

As growth of the bacteria occurs, antibiotic diffuses out

from each disc into the agar. If the concentration of the antibi-

otic is lethal, no growth of the bacteria will occur. Finally, the

diffusing antibiotic will be below lethal concentration, so that

growth of bacteria can occur. The result is a ring of no growth

around a disc. From comparison with known standards, the

diameter of the growth inhibition ring will indicate whether

the bacteria are resistant to the antibiotic.

Automated plate readers are available that will scan the

plates, measure the diameter of the growth inhibition zones

and consult a standard database to indicate the antibiotic

resistance or susceptibility of the sample bacteria.

In the past 15 years, the use of fluorescent indicators has

become popular. A myriad of compounds are available that

will fluoresce under illumination of specific wavelengths.

Among the uses for the fluorescent compounds is the viability

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