Microbiology and Immunology

Luminescent bacteria WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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of Berlin. By 1888, he had transferred to the University of

Greifswald where he spent the next 25 years.

At the University of Greifswald, Loeffler studied

Salmonella typhi-murium, the bacteriological agent that

causes mouse typhoid but does not infect other animals. This

research was intended to control the exuberant mouse popula-

tion that was threatening to destroy the crops of Greece.

Loeffler effectively killed the mice by contaminating their

food sources with the bacteria.

In 1898, Friedrich Loeffler, in conjunction with Paul

Frosch, determined a filterable agent proving smaller than

any bacteria previously discovered caused foot-and-mouth

disease. This was the first hint that virusesexisted. At that

time, Loeffler was working at the University of Greifswald as

head of the department of hygiene. Loeffler moved his labo-

ratory to the island of the Insel Riems in order to safely con-

tinue his research on the disease. In 1913, Loeffler’s research

took a back seat to his new position as director of the Robert

Koch Institute in Berlin. Once World War I began, all

research on the Insel Riems ceased. Loeffler worked for the

army to implement proper hygiene regimens until his death in

1915.

See alsoCoryneform bacteria; Streptococci and streptococcal

infections

LLuminescent bacteriaUMINESCENT BACTERIA

Luminescence is the emission of light by an object. Living

organisms including certain bacteriaare capable of lumines-

cence (bioluminescence). Bacteria are the most abundant

luminescent organism in nature.

Bacterial luminescence has been studied most exten-

sively in several marine bacteria (e.g., Vibrio harveyi, Vibrio

fischeri, Photobacterium phosphoreum, Photobacterium

leiognathi), and in Xenorhabdus luminescens, a bacteria that

lives on land. The precise molecular mechanisms of lumines-

cence differ between these bacteria. But, the general scheme

of the process is similar.

In luminescent bacteria (and other luminescent organ-

isms as well) this general scheme involves an enzyme that is

dubbed luciferase. A suite of genes dubbed lux genes code

for the enzyme and other components of the luminescent sys-

tem. The different bacteria are dissimilar in the sequence of

their lux genes and in the enzyme reactions that produce

luminescence. However, the general pattern of the reaction is

the same.

A similarity between the luminescent bacteria concerns

the conditions that prompt the luminescence. A key factor is the

number of bacteria that are present. This is also known as the

cell density (i.e., the number of bacteria per given volume of

solution or given weight of sample). A low cell density (e.g.,

less than 100 living bacteria per milliliter) does not induce

luminescence, whereas luminescence is induced at a high cell

density (e.g., 10^10 to 10^11 living bacteria per milliliter).

The effect of cell density is particularly evident in those

luminescent bacteria that live in the ocean. When living free

in the ocean water, Vibrio fischeri is not luminescent.

However, when living in a confined space such as the inside

of a fish or squid, Vibrio fischeriis luminescent. Bacterial

luminescence may have evolved as a means of enhancing the

survival of the bacteria species. For example, the lumines-

cence of Vibrio fischeriin a squid enables the squid to cam-

ouflage itself from undersea predators in the moonlit ocean.

In return for this protection, the squid provides the bacteria

with a hospitable environment.

The influence of cell density on bacterial luminescence

is due to the nature of the luminescent process. The bacteria

produce a chemical called homoserine lactone. At low cell

densities, the chemical exits a bacterium and drifts away in the

fluid that surround the cell. But at high cell densities when the

bacteria are tightly packed together, the homoserine lactone

stays in the immediate vicinity of the bacteria. Then, the

chemical is able to stimulate the activity of the lux genes that

are responsible for the luminescence. This occurs when the

homoserine lactone binds to a protein in the bacterial cyto-

plasmcalled LuxR. The LuxR-homoserine lactone complex

then binds to a region of the bacterial DNAthat is the master

control for the activity of the lux genes.

Bacterial luminescence is due to the action of the

enzyme called luciferase. Luciferase catalyses the removal of

an electron from two compounds. Excess energy is liberated in

this process. The energy is dissipated as a luminescent blue-

green light. Luminescent bacteria contain a number of genes

that are found linked to each other in the bacterial genome,

and which are controlled by a common regulatory region of

the DNA. This arrangement of genes is called an operon.

The lux genes are involved in the production of

luciferase, in the production and activity of the LuxR protein

that detects the homoserine lactone, and in the chemicals reac-

tions that produce the compounds on which the luciferase acts.

Bacteria utilize homoserine lactone in other cell-to-cell

communications that are cell-density dependent. One example

is the formation of the adherent, exopolysaccharide-enmeshed

populations, known as biofilms, by the bacterium

Pseudomonas aeruginosa. Another example is the bacterium

Agrobacteriumthat produces diseases in some plants. The

phenomenon has been termed quorum sensing.

The lux genesystem responsible for bacterial lumines-

cence has become an important research tool and commercial

product. The incorporation of the luminescent genes into other

bacteria allows the development of bacterial populations to be

traced visually. Because luminescence can occur over and

over again and because a bacterium’s cycle of luminescence is

very short (i.e., a cell is essentially blinking on and off), lumi-

nescence allows a near instantaneous (i.e., “real time”) moni-

toring of bacterial behavior. The use of lux genes is being

extended to eukaryotic cells. This development has created the

potential for the use of luminescence to study eukaryotic cell

density related conditions such as cancer.

See alsoBacterial adaptation; Bioluminescence; Economic

uses and benefits of microorganisms

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