Microbiology and Immunology

Bacteriocidal, bacteriostatic WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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A second technique of sample preparation relies on the

instantaneous freezing of the bacteria. Freezing is so fast that

the interior water does not extensively crystallize (which

would be extremely damaging to structure). Again, an experi-

enced analyst can produce samples that information concern-

ing the native ultrastructure of the bacteria.

In the past several decades, other tools are increasing

the ultrastructure information that can be obtained. For exam-

ple, the technique of atomic force microscopy can produce

information on the atomic associations between adjacent mol-

ecules on the surface of bacteria. Atomic force microscopy has

been very useful in ultrastructure studies of the regularly

structured surface layers on bacteria.

Modern techniques of molecular geneticscan also yield

ultrastructure information. Mutants can be selected or

designed in which a particular geneor genes has been ren-

dered incapable of producing a protein product. If the gene is

involved with cell wall constituents, the analysis of the wall

can reveal the alterations that have occurred in the absence of

the gene product. An example are the many mutants that are

defective in the construction or assembly of lipopolysaccha-

ride, a carbohydrate and lipid constituent of the outer mem-

brane of Gram-negative bacteria. The loss of the carbohydrate

portion of lipopolysaccharide makes the outer membrane

more hydrophobic.

One approach that has been known for decades still

yields useful information concerning bacterial ultrastructure.

This is the substitution of the metals present in the cell wall

with other metals. Metals act like glue to hold various wall

components in association with one another. Examples of such

metallic species include calcium and magnesium. Out-com-

peting these species by supplying large concentrations of

another metal, the influence of the normal metallic species can

be assessed. For example, replacement of metals in the Gram-

negative outer membrane can cause the release of lipopolysac-

charide and the formation of bubbles along the surface of the

membrane, where the underlying attachment to the rigid pep-

tidoglycanlayer is disrupted.

The use of specific antibodies to determine the molecu-

lar arrangement of ultrastructural constituent targets greatly

enhances the effectiveness of agents to be used in drug therapy.

See alsoAtomic force microscope; Bacterial appendages;

Bacterial surface layers; Caulobacter; Electron microscope,

transmission and scanning; Electron microscopic examination

of microorganisms; Sheathed bacteria

BACTERIOCHLOROPHYLL•seePHOTOSYNTHESIS

BBacteriocidal, bacteriostaticACTERIOCIDAL, BACTERIOSTATIC

Bacteriocidal is a term that refers to the treatment of a bac-

terium such that the organism is killed. Bacteriostatic refers to

a treatment that restricts the ability of the bacterium to grow.

A bacteriocidal treatment is always lethal and is also referred

to as sterilization. In contrast, a bacteriocidal treatment is nec-

essarily lethal.

Bacteriocidal methods include heat, filtration, radiation,

and the exposure to chemicals. The use of heat is a very pop-

ular method of sterilization in a microbiology laboratory. The

dry heat of an open flame incinerates microorganismslike

bacteria, fungiand yeast. The moist heat of a device like an

autoclave can cause deformation of the protein constituents of

the microbe, as well as causing the microbial membranes to

liquefy. The effect of heat depends on the time of exposure in

addition to form of heat that is supplied. For example, in an

autoclave that supplies a temperature of 121° F (49.4° C), an

exposure time of 15 minutes is sufficient to kill the so-called

vegetative form of bacteria. However, a bacterial spores can

survive this heat treatment. More prolonged exposure to the

heat is necessary to ensure that the spore will not germinate

into a living bacteria after autoclaving. The relationship

between the temperature and the time of exposure can be com-

puted mathematically.

A specialized form of bacteriocidal heat treatment is

called pasteurizationafter Louis Pasteur, the inventor of the

process. Pasteurization achieves total killing of the bacterial

population in fluids such as milk and fruit juices without

changing the taste or visual appearance of the product.

Another bacteriocidal process, albeit an indirect one, is

filtration. Filtration is the physical removal of bacteria from a

fluid by the passage of the fluid through the filter. The filter

contains holes of a certain diameter. If the diameter is less than

the smallest dimension of a bacterium, the bacterium will be

retained on the surface of the filter it contacts. The filtered

fluid is sterile with respect to bacteria. Filtration is indirectly

bactericidal since the bacteria that are retained on the filter

will, for a time, be alive. However, because they are also

removed from their source of nutrients, the bacteria will even-

tually die.

Exposure to electromagnetic radiation such as ultravio-

let radiation is a direct means of killing bacteria. The energy

of the radiation severs the strands of deoxyribonucleic acidin

many locations throughout the bacterial genome. With only

one exception, the damage is so severe that repair is impossi-

ble. The exception is the radiation resistant bacterial genus

called Deinococcus. This genus has the ability to piece

together the fragments of DNAin their original order and enzy-

matic stitch the pieces into a functional whole.

Exposure to chemicals can be bacteriocidal. For exam-

ple, the gas ethylene oxide can sterilize objects. Solutions con-

taining alcohol can also kill bacteria by dissolving the

membrane(s) that surround the contents of the cell. Laboratory

benches are routinely “swabbed” with an ethanol solution to

kill bacteria that might be adhering to the bench top. Care

must be taken to ensure that the alcohol is left in contact with

the bacteria for a suitable time (e.g., minutes). Otherwise, bac-

teria might survive and can even develop resistance to the bac-

tericidal agent. Other chemical means of achieving bacterial

death involve the alteration of the pH, salt or sugar concentra-

tions, and oxygen level.

Antibioticsare designed to be bacteriocidal. Penicillin

and its derivatives are bactericidal because they act on the pep-

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