Microbiology and Immunology

Pasteurella WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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become active. He hypothesized that if people were given an

injection of a vaccine after being bitten, it could prevent the

disease from manifesting. After methodically producing a

rabies vaccine from the spinal fluid of infected rabbits, Pasteur

sought to test it. In 1885, nine-year-old Joseph Meister, who

had been bitten by a rabid dog, was brought to Pasteur, and

after a series of shots of the new rabies vaccine, the boy did

not develop any of the deadly symptoms of rabies.

To treat cases of rabies, the Pasteur Institute was estab-

lished in 1888 with monetary donations from all over the

world. It later became one of the most prestigious biological

research institutions in the world. When Pasteur died in 1895,

he was well recognized for his outstanding achievements in

science.

See alsoBacteria and bacterial infection; Colony and colony

formation; Contamination, bacterial and viral; Epidemiology,

tracking diseases with technology; Epidemiology; Food

preservation; Germ theory of disease; History of microbiol-

ogy; History of public health; Immunogenetics; Infection con-

trol; Winemaking

PPasteurellaASTEURELLA

Pasteurellais a genus, or subdivision, of bacteria. The genus

is in turn a member of the family Pasteurellaceae, which

includes the genus Hemophilus. Members of this genus

Pasteurellaare short rod-shaped bacteria that produce the neg-

ative reaction in the Gram stain procedure, are incapable of the

active type of movement called motility, and can grow both in

the presence and the absence of oxygen.

Pasteurella causes diseases in humans and many

species of animals. One species in particular, Pasteurella mul-

tocidacauses disease in both humans and animals. For exam-

ple, almost all pet rabbits will at one time or another acquire

infections of the nose, eyes, and lungs, or develop skin sores

because of a Pasteurella multocidainfection. The bacterium

also causes a severe infection in poultry, including lameness

and foul cholera, and illness in cattle and swine. Another

species, Pasteurella pneumotrophica, infects mice, rats,

guinea pigs, hamsters, and other animals that are often used in

laboratory studies.

The annual economic cost of the losses due to

these infections are several hundred million dollars in the

United States alone.

In humans, Pasteurella multocidacan be acquired from

the bite of a cat or dog. From 20% to 50% of the one to two

million Americans, mostly children, who are bitten by dogs

and cats each year will develop the infection. Following some

swelling at the site of the bite, the bacteria can migrate. An

infection becomes established in nearby joints, where it pro-

duces swelling, arthritis, and pain.

Infections respond to common antibioticsincluding

penicillin, tetracycline, and chloramphenicol. Despite the rela-

tive ease of treatment of the infection, little is still known of

the genetic basis for the ability of the bacteria to establish an

infection, and of the factors that allow the bacterium to evade

the defense mechanisms of the host. In the controlled condi-

tions of the laboratory, the adherent populations known as

biofilms can be formed by Pasteurella multocida.

The recent completion of the genetic sequence of

Pasteurella multocidawill aid in determining the genes, and

so their protein products, which are critical for infection.

See alsoBacteria and bacterial infection; Proteomics

PPasteurizationASTEURIZATION

Pasteurization is a process whereby fluids such as wine and

milk are heated for a predetermined time at a temperature that

is below the boiling point of the liquid. The treatment kills any

microorganismsthat are in the fluid but does not alter the

taste, appearance, or nutritive value of the fluid.

The process of pasteurization is named after the French

chemist Louis Pasteur(1822–1895), who is regarded as the

founder of the study of modern microbiology. Among

Pasteur’s many accomplishments was the observation that the

heating of fluids destroys harmful bacteria.

The basis of pasteurization is the application of heat.

Many bacteria cannot survive exposure to the range of temper-

atures used in pasteurization. The energy of the heating process

is disruptive to the membrane(s) that enclose the bacteria. As

well, the bacterial enzymesthat are vital for the maintenance of

the growth and survival of the bacteria are denatured, or lose

their functional shape, when exposed to heat. The disruption of

bacteria is usually so complete that recovery of the cells fol-

lowing the end of the heat treatment is impossible.

The pasteurization process is a combination of temper-

ature, time, and the consistency of the product. Thus, the

actual conditions of pasteurization can vary depending on the

product being treated. For example heating at 145°F (63°C)

for not less than 30 minutes or at 162°F (72°C) for not less

than 16 seconds pasteurizes milk. A product with greater con-

sistency, such ice cream or egg nog, is pasteurized by heating

at a temperature of at least 156°F (69°C) for not less than 30

minutes or at a temperature of at least 176°F (80°C) for not

less than 25 seconds.

Particularly in commercial settings, such as a milk pro-

cessing plant, there are two long-standing methods of pasteur-

ization. These are known as the batch method and the

continuous method. In the batch method the fluid is held in

one container throughout the process. This method of pasteur-

ization tends to be used for products such as ice cream. Milk

tends to be pasteurized using the continuous method.

In the continuous method the milk passes by a stack of

steel plates that are heated to the desired temperature. The

flow rate is such that the milk is maintained at the desired tem-

perature for the specified period of time. The pasteurized milk

then flows to another tank.

Several other more recent variations on the process of

pasteurization have been developed. The first of these varia-

tions is known as flash pasteurization. This process uses a

higher temperature than conventional pasteurization, but the

temperature is maintained for a shorter time. The product is

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