Hugo W.B., Russel A.D.(ed). Pharmaceutical Microbiology
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the plant kingdom, but they are now called prokaryotes, a name which means primitive
nucleus. All other living organisms are called eukaryotes, a name implying a true or
proper nucleus. This important division does not invalidate classification schemes within
the world of bacterial, animal and plant life.
This subdivision is not based on the more usual macroscopic criteria; it was made
possible when techniques of subcellular biology became sufficiently refined for many
more fundamental differences to become apparent. Some of the criteria differentiating
eukaryotes and prokaryotes are given in Table 1.1.
Recently, a third class must be added to the bacteria and blue-green algae. Organisms
in this class have been named the Archaebacteria; they differ from bacteria and blue-
green algae in their wall and membrane structure and pattern of metabolism. They are
thought by many to be the first living organisms to have appeared on earth.
2 Structure and form of the bacterial cell
2.1 Size and shape
The majority of bacteria fall within the general dimensions of 0.75-4(Xm. They are
unicellular structures which may occur as cylindrical (rod-shaped) or spherical (coccoid)
forms. In one or two genera, the cylindrical form may be modified in that a single twist
(vibrios) or many twists like a corkscrew (spirochaetes) may occur.
Another feature of bacterial form is the tendency of coccoid cells to grow in
aggregates. Thus, there exist assemblies (i) of pairs (called diplococci); (ii) of groups
of four arranged in a cube (sarcinae); (iii) in a generally unorganized array like a
bunch of grapes (staphylococci); and (iv) in a chain like a string of beads (streptococci).
The aggregates are often so characteristic as to give rise to the generic name of a
group, e.g. Diplococcus (now called Streptococcus) pneumoniae, a cause of pneumonia;
Staphylococcus aureus, a cause of boils and food poisoning; and Streptococcus pyogenes,
a cause of sore throat.
Rod-shaped organisms occasionally occur in chains either joined end to end or
branched.
2.2 Structure
Three fundamental divisions of the bacterial cell occur in all species: cell wall, cell or
cytoplasmic membrane, and cytoplasm.
4 Chapter 1
Table 1.1 The main features distinguishing prokaryotic and eukaryotic cells
Feature
Nucleus
Cell wall
Mitochondria
Mesosomes
Chloroplasts
Prokaryotes Eukaryotes
No enclosing membrane Enclosed by a membrane
Peptidoglycan Cellulose
Absent Present
Present Absent
Absent Present
2.2.7 Cell wall
Extensive chemical studies have revealed a basic structure of alternating 7V-
acetylglucosamine and A^-acetyl-3-O-l-carboxyethyl-glucosamine molecules, giving
a polysaccharide backbone. This is then cross-linked by peptide chains, the nature of
which varies from species to species. This structure (Fig. 1.1) possesses great mechanical
strength and is the target for a group of antibiotics which, in different ways, inhibit the
biosynthesis occurring during the cell growth and division (Chapter 8).
This basic peptidoglycan (sometimes called murein or mucopeptide) also contains
other chemical structures which differ in two types of bacteria, Gram-negative and
Gram-positive. In 1884, Christian Gram discovered a staining method for bacteria which
bears his name. It consists of treating a film of bacteria, dried on a microscope slide,
with a solution of a basic dye, such as gentian violet, followed by application of a
solution of iodine. The dye complex may be easily washed from some types of cells
which, as a result, are called Gram-negative whereas others, termed Gram-positive,
retain the dye despite alcohol washing. These marked differences in behaviour,
discovered by chance, are now known to be a reflection of different wall structures in
the two types of cell. These differences reside in the differing chemistry of material
attached to the outside of the peptidoglycan (Fig. 1.2).
In the walls of Gram-positive bacteria, molecules of a polyribitol or polyglycerol-
phosphate are attached by covalent links to the oligosaccharide backbone; these entities
Fig. 1.2 A, peptidoglycan of Escherichia coli. •, /V-acetylmuramic acid; •, Af-acetylglucosamine. B,
repeating unit of peptidoglycan of E. coli. L-ala, L-alanine; D-glu, D-glutamine; DAP, diaminopimelic
acid: D-aia, D-alanine.
Fig. 1.3 A, glycerol teichoic
acid; B, ribitol teichoic acid; G,
glycosyl; Ala, D-alanyl.
Lipopolysaccharide
Cytoplasmic membrane
Fig. 1.4 Diagram showing detailed structure of the envelope of Gram-negative bacteria.
are teichoic acids (Fig. 1.3A, B). The glycerol teichoic acid may contain an alanine
residue (Fig. 1.3A). Teichoic acids do not confer additional rigidity on the cell wall,
but as they are acidic in nature they may function by sequestering essential metal cations
from the media on which the cells are growing. This could be of value in situations
where cation concentration in the environment is low.
The Gram-negative cell envelope (Fig. 1.4) is even more complicated; essentially,
it contains lipoprotein molecules attached covalently to the oligosaccharide backbone
and in addition, on its outer side, a layer of lipopolysaccharide (LPS) and protein attached
by hydrophobic interactions and divalent metal cations, Ca
2+
and Mg
2+
. On the inner
side is a layer of phospholipid (PL).
The LPS molecule consists of three regions, called lipid A, core polysaccharide
and O-specific side chain (Fig. 1.5). The O-specific side chain comprises an array of
sugars that are responsible for specific serological reactions of organisms, which are
used in identification. The lipid A region is responsible for the toxic and pyrogenic
(fever-producing) properties of this group (see Chapter 18).
The complex outer layers beyond the peptidoglycan in the Gram-negative species,
the outer membrane, protect the organism to a certain extent from the action of toxic
chemicals (see Chapter 13). Thus, disinfectants are often effective only at concentrations
higher than those affecting Gram-positive cells and these layers provide unique
protection to the cells from the action of benzylpenicillin and lysozyme.
Fig. 1.5 Lipopolysaccharide structure in Gram-negative bacteria.
Bacteria 7
Part of the LPS may be removed by treating the cells with ethylenediamine tetra-
acetic acid (EDTA) or related chelating agents (Chapter 12).
The proteins of the outer membrane, many of which traverse the whole structure,
are currently the subject of active study. Some of the proteins consist of three subunits,
and these units with a central space or pore running through them are known as porins.
They are thought to act as a mechanism of selectivity for the ingress or exclusion of
metabolites and antibacterial agents (see Chapter 8).
2.2.2 Cytoplasmic membrane
The chemistry and structure of this organelle have been the subject of more than a
century of research, but it is only during the last 20 years that some degree of finality
has been realized.
Chemically, the membrane is known to consist of phospholipids and proteins, many
of which have enzymic properties. The phospholipid molecules are arranged in a
bimolecular layer with the polar groups directed outwards on both sides. The structures
of some phospholipids found in bacteria are shown in Fig. 1.6. Earlier views held that
the protein part of the membrane was spread as a continuous sheet on either side of the
Fig. 1.6 The structure of some
phospholipids found in E. coli.
A, the structure of phosphatidic
acid. H* of this structure is
replaced by grouping B-D to
give the following phospholipids:
B, phosphatidylethanolamine; C,
phosphatidylglycerol; D,
diphosphatidylglycerol
(cardiolipin). R
A
.COO and
R
B
.COO are fatty acid residues.
Fig. 1.7 Membrane structure.
phospholipid bilayer. The current view is that protein is distributed in local patches in
the bilayer, the mosaic structure (Fig. 1.7).
Unlike the wall, which has great mechanical strength, determines the characteristic
shape of the cell and is metabolically inert, the membrane is structurally a very delicate
organelle and is highly active metabolically.
The membrane acts as a selective permeability barrier between the cytoplasm and
the cell environment; the wall acts only as a sieve to exclude molecules larger than
about 1 nm. Certain enzymes, and especially the electron transport chain, that are located
in the membrane are responsible for an elaborate active transport system which utilizes
the electrochemical potential of the proton to power it.
An interesting experiment serves to illustrate the differing mechanical strengths of
the wall and membrane. The wall of some Gram-positive bacteria may be partially
dissolved by treatment of cells with lysozyme or in the case of Gram-negative cells
with EDTA plus lysozyme. Upon doing this, cells so treated burst due to the fact that
the cytoplasm contains a large number of solutes giving it an effective osmotic pressure
of 608-2533 kPa (6-25 atm). Water enters the cell, now no longer protected by the
peptidoglycan, causing the naked protoplast to swell and burst. If this experiment is
conducted in a medium containing 0.33 M sucrose, a non-penetrating solute, the osmotic
pressure inside and outside the protoplast is equalized, thus no bursting occurs, and
forms free of cell wall (protoplasts) may be observed in the medium.
2.2.3 Cytoplasm
The cytoplasm is a viscous fluid and contains within it systems of paramount im-
portance. These are the nucleus, responsible for the genetic make-up of the cell, and
the ribosomes, which are the site of protein synthesis. In addition are found granules of
reserve material such as polyhydroxybutyric acid, an energy reserve, and polyphosphate
or volutin granules, the exact function of which has not yet been elucidated. The
prokaryotic nucleus or bacterial chromosome exists in the cytoplasm in the form of a
loop and is not surrounded by a nuclear membrane. Bacteria carry other chromosomal
elements: episomes, which are portions of the main chromosome that have become
isolated from it, and plasmids, which may be called miniature chromosomes. These are
small annular pieces of DNA which carry a limited amount of genetic information,
Bacteria 9
Protein
often associated with the expression of resistance to antimicrobial agents (Chapters 9
and 13).
Despite the differences in nuclear structures between prokaryotes and eukaryotes,
the genetic code, i.e. the combination of bases which does for a particular amino acid
in the process of protein synthesis, is the same as it is in all living organisms.
2.2.4 Appendages to the bacterial cell
Three types of thread-like appendages may be found growing from bacterial cells:
flagella, pili (fimbriae) and F-pili (sex strands).
Flagella are threads of protein often \2fim. long which start as small basal organs
just beneath the cytoplasmic membrane. They are responsible for the movement of
motile bacteria. Their number and distribution varies. Some species bear a single
flagellum, others are flagellate over their whole surface.
Pili are responsible for haemagglutination in bacteria and also for intercellular
adhesiveness giving rise to clumping. At present, a clear role for these structures has
not been formulated.
F-pili or sex strands are part of a primitive genetic exchange system in some bacterial
species. Part of the genetic material may be passed from one cell to another through the
hollow pilus, thus giving rise to a simple form of sexual reproduction.
2.2.5 Capsules and slime
Some bacterial species accumulate material as a coating of varying degrees of looseness.
If the material is reasonably discrete it is called a capsule, if loosely bound to the
surface it is called slime.
Recently a phenomenon of resistance to biocide solutions has been recognized
(see also Chapters 9 and 13) in which bacteria adhere to a container wall and cover
themselves with a carbohydrate slime called a glycocalyx; thus, doubly protected (wall
and glycocalyx), they have been found to resist biocide attack.
Bacillus anthracis, the causative organism of anthrax, possesses a capsule composed
of polyglutamic acid; the slime layers produced by other organisms are of a carbohydrate
nature.
An extreme example of slime production is found in Leuconostoc dextranicum
and L. mesenteroides where so much carbohydrate, called dextran, may be produced
that the whole medium in which these cells are growing becomes almost gel-like. This
phenomenon has caused pipe blockage in sugar refineries and is deliberately encouraged
for the production of dextran as a blood substitute (Chapter 25).
2.2.6 Pigments
Some bacterial species produce pigments during their growth which give the colonies
a characteristic colour.
Thus, Staphylococcus aureus produces a golden yellow pigment, Serratia marcescens
a bright red pigment. There appears to be no valid function for these pigments but they
may afford the cell some protection from the toxic effects of sunlight.
10 Chapter 1
spore. spore core
2.3 The bacterial spore
In a few bacterial genera, notably Bacillus and Clostridium, a unique process takes
place in which the vegetative cell undergoes a profound biochemical change to give
rise to a structure called a spore or endospore (Fig. 1.8). This process is not part of
a reproductive cycle, but the bacterial endospore is highly resistant to adverse
environments such as lack of moisture or essential nutrients, toxic chemicals and
radiations and high temperatures. Because of their heat resistance all sterilization
processes have to be designed to destroy the bacterial spore.
2.3.1 The process of spore formation
In general, an adverse environment, and in particular the absence or limited presence
of one component, induces spore formation. Examples of such components are alanine,
Zn
2+
, Fe
2+
, POj~ and, in the case of the aerobic (oxygen-requiring) Bacillus species,
oxygen. Equally, certain substances, for instance Ca
2+
and Mn
2+
, have to be present for
the process of spore formation to proceed to completion.
If the conditions for spore formation are fulfilled the sequence of events shown
in Fig. 1.9 occurs.
The essential genetic material of the original vegetative bacterium is retained in the
core or protoplast; around this lies the thick cortex which contains the murein or
peptidoglycan already encountered as a cell wall component (see Fig. 1.2). The outer
coats which are protein in composition are distinguished by their high cysteine content.
In this respect they resemble keratin, the protein of hair and horn.
Another feature of the spore is the presence of pyridine 2,6-dicarboxylic acid (DPA)
(Fig. 1.10) occurring as a complex with calcium, which at one time was implicated
in heat resistance. The isolation of heat-resistant spores containing no Ca-DPA has
refuted this hypothesis.
The reason for heat resistance is thought to lie in the fact that the core or spore
cytoplasm becomes dehydrated during sporulation. The mechanism for this dehydration
Bacteria 11
Growth" Formation ••• Formation Formation Release
of septum of cortex of coats of spore
Onset of radiation
resistance
Synthesis of cysteine-
rich structures
Synthesis of
spore-specific
proteins
Increased production of
enzymes required for
energy-yielding reactions
and for synthesis of RNA
and proteins
Antibiotics .
synthesized
Lytic enzymes
excreted
Maturation
of the
fully formed
spore
Onset of resistance
to heat
Synthesis of
dipicolinic acid
Incorporation
of
calcium
Retractile spores
visible in optical
microscope
Approximate time (hours) following commencement of sporulation
Fig. 1.9 Changes occurring during spore formation. The position and length of the boxes represent
the approximate time and duration of the various activities.
COOH COOH Fig. 1.10 Pyridine 2,6-dicarboxylic acid, dipicolinic acid (DPA).
is the mechanical expulsion of water by the expansion of the peptidoglycan network
which comprises the cortex—the expanded cortex theory.
Dehydration of the core by means of concentrated sucrose solution also results in
heat resistance.
The tough keratin-like spore coats probably help to protect the spore core or
protoplast from the harmful effects of chemicals. Radiation resistance has not been
fully explained.
The same generally impervious properties make spores difficult to stain by simple
stains. However, if a slide preparation of spores is warmed with a stain the spores are
dyed so effectively that dilute acid will not wash out the colour. This is the basis of the
acid-fast stain for spores.
2.3.2 Spore germination and outgrowth
In nature, spores can revert to the vegetative form by a process called either 'germination'
or 'germination and outgrowth'. The process of germination may be triggered by specific
12 Chapter 1
Inhibitors:
Fall in optical
density and
retractility RNA synthesis
0 5 min 1 hour 3 hours
Approximate time following initiation of germination
Fig. 1.11 Spore germination, outgrowth and division of the outgrown cells.
germination stimulants, such as L-alanine or glucose, by the physical processes of
shaking with small glass beads, or by sublethal heating (e.g. at 60°C for 1 hour).
Outgrowth and subsequent growth depends on the presence of the necessary nutrients
for the particular organism concerned. The stages of germination and outgrowth, and
also the action of inhibitors of the process, are shown in Fig. 1.11.
2.3.3
Parameters of heat resistance
The existence and possible presence of bacterial spores determines the parameters,
i.e. time and temperature relationships, of thermal sterilization processes which are
used extensively by the food and pharmaceutical industry. These are defined below
(see also Chapters 20 and 23).
1 D-value (decimal reduction time, DRT) is the time in minutes required to destroy
90% of a population of cells. The D-value has little relevance to the sterilization of
medicines for injection, surgical instruments or dressings, where a process designed to
kill all living spores must be developed. The D-value is used extensively in the food
industry.
2 The F
Q
-value is a process-describing unit expressed in terms of minutes at 121.1 °C
(originally 250°F) or a corresponding time-temperature relationship to produce the
same complete spore-killing effect.
3 The z-value is the increase in temperature (°C) to reduce the D-value to
one-tenth.
Bacteria 13