WORLD OF MICROBIOLOGY AND IMMUNOLOGY Phage genetics433•
See alsoAntibody and antigen; Antibody formation and kinet-
ics; Bacteria and bacterial infection; Bactericidal, bacteriosta-
tic; Bubonic plague; Epidemics, bacterial; Infection and
resistance; Meningitis, bacterial and viral; Pseudomonas;
Serology; Typhoid fever; TyphusPpHH
The term pH refers to the concentration of hydrogen ions (H+)
in a solution. An acidic environment is enriched in hydrogen
ions, whereas a basic environment is relatively depleted of
hydrogen ions. The pH of biological systems is an important
factor that determines which microorganism is able to survive
and operate in the particular environment. While most
microorganismsprefer pH’s that approximate that of distilled
water, some bacteriathrive in environments that are extremely
acidic.
The hydrogen ion concentration can be determined
empirically and expressed as the pH. The pH scale ranges
from 0 to 14, with 1 being the most acidic and 14 being the
most basic. The pH scale is a logarithmic scale. That is, each
division is different from the adjacent divisions by a factor of
ten. For example, a solution that has a pH of 5 is 10 times as
acidic as a solution with a pH of 6.
The range of the 14-point pH scale is enormous.
Distilled water has a pH of 7. A pH of 0 corresponds to 10 mil-
lion more hydrogen ions per unit volume, and is the pH of bat-
tery acid. A pH of 14 corresponds to one ten-millionth as many
hydrogen ions per unit volume, compared to distilled water,
and is the pH of liquid drain cleaner.
Compounds that contribute hydrogen ions to a solution
are called acids. For example, hydrochloric acid (HCl) is a
strong acid. This means that the compounds dissociates easily
in solution to produce the ions that comprise the compound
(H+and Cl–). The hydrogen ion is also a proton. The more pro-
tons there are in a solution, the greater the acidity of the solu-
tion, and the lower the pH.
Mathematically, pH is calculated as the negative loga-
rithm of the hydrogen ion concentration. For example, the
hydrogen ion concentration of distilled water is 10–7and hence
pure water has a pH of 7.
The pH of microbiological growth media is important in
ensuring that growth of the target microbes occurs. As well,
keeping the pH near the starting pH is also important, because
if the pH varies too widely the growth of the microorganism
can be halted. This growth inhibition is due to a numbers of
reasons, such as the change in shape of proteins due to the
presence of more hydrogen ions. If the altered protein ceases
to perform a vital function, the survival of the microorganism
can be threatened. The pH of growth media is kept relatively
constant by the inclusion of compounds that can absorb excess
hydrogen or hydroxyl ions. Another means of maintaining pH
is by the periodic addition of acid or base in the amount
needed to bring the pH back to the desired value. This is usu-
ally done in conjunction with the monitoring of the solution,
and is a feature of large-scale microbial growth processes,
such as used in a brewery.Microorganisms can tolerate a spectrum of pHs.
However, an individual microbe usually has an internal pH
that is close to that of distilled water. The surrounding cell
membranes and external layers such as the glycocalyxcon-
tribute to buffering the cell from the different pH of the sur-
rounding environment.
Some microorganisms are capable of modifying the pH
of their environment. For example, bacteria that utilize the
sugar glucose can produce lactic acid, which can lower the pH
of the environment by up to two pH units. Another example is
that of yeast. These microorganisms can actively pump hydro-
gen ions out of the cell into the environment, creating more
acidic conditions. Acidic conditions can also result from the
microbial utilization of a basic compound such as ammonia.
Conversely, some microorganisms can raise the pH by the
release of ammonia.
The ability of microbes to acidify the environment has
been long exploited in the pickling process. Foods commonly
pickled include cucumbers, cabbage (i.e., sauerkraut), milk
(i.e., buttermilk), and some meats. As well, the production of
vinegar relies upon the pH decrease caused by the bacterial
production of acetic acid.See alsoBiochemistry; Buffer; ExtremophilesPPhage geneticsHAGE GENETICS
Bacteriophages, virusesthat infect bacteria, are useful in the
study of how genes function. The attributes of bacteriophages
include their small size and simplicity of genetic organization.
The most intensively studied bacteriophageis the phage
called lambda. It is an important model system for the latent
infection of mammalian cells by retroviruses, and it has been
widely used for cloningpurposes. Lambda is the prototype of
a group of phages that are able to infect a cell and redirect the
cell to become a factory for the production of new virus parti-
cles. This process ultimately results in the destruction of the
host cell (lysis). This process is called the lytic cycle. On the
other hand, lambda can infect a cell, direct the integration of
its genome into the DNAof the host, and then reside there.
Each time the host genome replicates, the viral genome under-
goes replication, until such time as it activates and produces
new virus particles and lysis occurs. This process is called the
lysogenic cycle.
Lambda and other phages, which can establish lytic or
lysogenic cycles, are called temperate phages. Other examples
of temperate phages are bacteriophage mu and P1. Mu inserts
randomly into the host chromosome causing insertional muta-
tionswhere intergrations take place. The P1 genome exists in
the host cell as an autonomous, self-replicating plasmid.
Phage geneexpression during the lytic and lysogenic
cycles uses the host RNApolymerase, as do other viruses.
However, lambda is unique in using a type of regulation called
antitermination.
As host RNA polymerase transcribes the lambda
genome, two proteins are produced. They are called cro (for
“control of repressor and other things”) and N. If the lyticwomi_P 5/7/03 11:09 AM Page 433