WORLD OF MICROBIOLOGY AND IMMUNOLOGY Pertussis429•
PPeptidoglycanEPTIDOGLYCAN
Peptidoglycan is the skeleton of bacteria. Present in both
Gram-positive and Gram-negative bacteria, the peptidoglycan
is the rigid sac that enables the bacterium to maintain its shape.
This rigid layer is a network of two sugars that are
cross-linked together by amino acid bridges. The sugars are N-
acetyl glucosamine and N-acetyl muramic acid. The latter
sugar is unique to the peptidoglycan, and is found no where
else in nature.
The peptidoglycan in Gram-negative bacteria is only a
single layer thick, and appears somewhat like the criss-cross
network of strings on a tennis racket. The layer lies between
the two membranes that are part of the cell wall of Gram-neg-
ative bacteria, and comprises only about twenty percent of the
weight of the cell wall. In Gram-positive bacteria, the pepti-
doglycan is much thicker, some 40 sugars thick, comprising
up to ninety percent of the weight of the cell wall. The cross
bridging is three-dimensional in this network. The peptidogly-
can layer is external to the single membrane, and together they
comprise the cell wall of Gram-positive bacteria.
Research has demonstrated that the growth of the pepti-
doglycan occurs at sites all over a bacterium, rather than at a
single site. Newly made peptidoglycan must be inserted into
the existing network in such a way that the strength of the pep-
tidoglycan sheet is maintained. Otherwise, the inner and outer
pressures acting on the bacterium would burst the cell. This
problem can be thought of as similar to trying to incorporate
material into an inflated balloon without bursting the balloon.
This delicate process is accomplished by the coordinate action
of enzymesthat snip open the peptidoglycan, insert new mate-
rial, and bind the old and new regions together. This process is
also coordinated with the rate of bacterial growth. The faster a
bacterium is growing, the more quickly peptidoglycan is made
and the faster the peptidoglycan sac is enlarged.
Certain antibioticscan inhibit the growth and proper
linkage of peptidoglycan. An example is the beta-lactam class
of antibiotics (such as penicillin). Also, the enzyme called
lysozyme, which is found in the saliva and the tears of
humans, attacks peptidoglycan by breaking the connection
between the sugar molecules. This activity is one of the impor-
tant bacterial defense mechanisms of the human body.See alsoBacterial ultrastructurePPeriplasmERIPLASM
The periplasm is a region in the cell wall of Gram-negative
bacteria. It is located between the outer membrane and the
inner, or cytoplasmic, membrane. Once considered to be
empty space, the periplasm is now recognized as a specialized
region of great importance.
The existence of a region between the membranes of
Gram-negative bacteria became evident when electron micro-
scopic technology developed to the point where samples
could be chemically preserved, mounted in a resin, and sliced
very thinly. The so-called thin sections allowed electrons topass through the sample when positioned in the electron
microscope. Areas containing more material provided more
contrast and so appeared darker in the electron image. The
region between the outer and inner membranes presented a
white appearance. For a time, this was interpreted as being
indicative of a void. From this visual appearance came the
notion that the space was functionless. Indeed, the region was
first described as the periplasmic space.
Techniques were developed that allowed the outer
membrane to be made extremely permeable or to be removed
altogether while preserving the integrity of the underlying
membrane and another stress-bearing structure called the pep-
tidoglycan. This allowed the contents of the periplasmic space
to be extracted and examined.
The periplasm, as it is now called, was shown to be a
true cell compartment. It is not an empty space, but rather is
filled with a periplasmic fluid that has a gel-like consistency.
The periplasm contains a number of proteins that perform var-
ious functions. Some proteins bind molecules such as sugars,
amino acids, vitamins, and ions. Via association with other
cytoplasmic membrane-bound proteins these proteins can
release the bound compounds, which then can be transported
into the cytoplasmof the bacterium. The proteins, known as
chaperons, are then free to diffuse around in the periplasm and
bind another incoming molecule. Other proteins degrade large
molecules such as nucleic acid and large proteins to a size that
is more easily transportable. These periplasmic proteins
include proteases, nucleases, and phosphatases. Additional
periplasmic proteins, including beta lactamase, protect the
bacterium by degrading incoming antibioticsbefore they can
penetrate to the cytoplasm and their site of lethal action.
The periplasm thus represents a buffer between the
external environment and the inside of the bacterium. Gram-
positive bacteria, which do not have a periplasm, excrete
degradative enzymesthat act beyond the cell to digest com-
pounds into forms that can be taken up by the cell.See alsoBacterial ultrastructure; Chaperones; PorinsPPertussisERTUSSIS
Pertussis, commonly known as whooping cough, is a highly
contagious disease caused by the bacteriaBordatella pertus-
sis. It is characterized by classic paroxysms (spasms) of
uncontrollable coughing, followed by a sharp intake of air
which creates the characteristic “whoop” from which the
name of the illness derives.
B. pertussisis uniquely a human pathogen (a disease
causing agent, such as a bacteria, virus, fungus, etc.) mean-
ing that it neither causes disease in other animals, nor sur-
vives in humans without resulting in disease. It exists
worldwide as a disease-causing agent, and causes epidemics
cyclically in all locations.
B. pertussiscauses its most severe symptoms by attack-
ing specifically those cells in the respiratory tract which have
cilia. Cilia are small, hair-like projections that beat constantly,
and serve to constantly sweep the respiratory tract clean ofwomi_P 5/7/03 11:08 AM Page 429