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Term Paper on Bacteria
Term Paper Contents:
- Term Paper on the Definition of Bacteria
- Term Paper on the Features of Bacteria
- Term Paper on the Path of Infection in Bacteria
- Term Paper on the Bacterial Waste Products
- Term Paper on the Shape and Size of Bacteria
- Term Paper on the Classification of Bacteria
- Term Paper on the Requirements for Bacterial Growth
- Term Paper on the Importance of the Bacterial Cell Wall
- Term Paper on the Transmission of Bacterial Infection
- Term Paper on the Pathogenesis of Bacterial Diseases
Term Paper # 1. Definition of Bacteria:
Bacteria are amongst the most successful living organisms. Their ubiquity ensures that human are obliged to live in constant and intimate contact with a wide variety of species and to encounter, if briefly, many more. Fortunately, relatively few species routinely cause disease (the so-called pathogenic bacteria) but many others have the potential to do so, given appropriate conditions.
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Whether or not a bacterial encounter leads to disease is determined by the balance of two principal factors-host factors, including the state of the individual’s immune system and features of the bacterium that enable it to cause disease. These bacterial features are often termed virulence determinants.
Virulence determinants enable bacteria to: compete successfully with the normal microflora; survive in adverse conditions; adhere to or enter their targeted cells; and evade defence mechanisms.
Term Paper # 2. Features of Bacteria:
Bacteria are prokaryotes, that is they lack an organized nucleus. Their genetic information is carried in a double-stranded, circular molecule of DNA which is often referred to as a chromosome although it differs from eukaryotic chromosomes in that no introns (non-coding sequences of DNA) are present.
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Some bacteria possess small circular extra-chromosomal DNA fragments known as plasmids which replicate independently of the chromosomal DNA. Plasmids may contain important genes coding for virulence factors or antibiotic resistance and may be transferred from one bacterium to another. The cytoplasm of bacteria contains many ribosomes but no mitochondria or other organelles.
In all bacteria, the cell is surrounded by a complex cell wall. The nature of the cell wall is important in the classification of bacteria and is determining virulence.
Term Paper # 3. Path of Infection in Bacteria:
Bacteria gain entrance to the body by various routes. Some enter through the broken skin (occasionally through the unbroken skin), some by way of the respiratory passages, others by way of the alimentary tract. The portal of entry determines whether or not pathogenic bacteria are capable of producing an infection.
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The organism of typhoid fever would probably not produce an infection if rubbed into the broken skin but if swallowed may reach the intestinal tract and produce the disease. The organism of gas gangrene will have no effect if swallowed but may produce a fatal infection if rubbed into the broken skin. Therefore, bacteria must enter the body by the route to which they are adapted.
However, this is not the only factor that determines whether or not an infection will result. Man and animals possess several defense mechanisms for destroying invading bacteria. If these mechanisms are vigorous and very active, they will usually defend the host against the disease organisms. On the other hand, if they are below normal and the invaders are very virulent, an infection may occur.
After bacteria invade the tissues, they may attack the host in a variety of ways. The organisms may produce a local inflammation or may localize in the liver, bone marrow, spleen, lymph glands, or other places, giving rise to secondary abscesses or secondary focuses of infection, also known as metastatic infections. Sometimes, organisms invade the blood stream, producing a bacteremia or septicemia (blood poisoning).
Term Paper # 4. Bacterial Waste Products:
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Bacteria produce a large number of waste products in the culture medium in which they are growing. The formation of some of these compounds is dependent upon the presence of certain specific precursors in the culture medium. The formation of others is not dependent upon the composition of the medium but is a characteristic of the organisms themselves. The composition of the medium merely determines whether the compounds shall be produced in larger or smaller amounts.
To the former group belong such compounds as the ptomaines (amines), indole, skatole, phenol, and hydrogen sulfide. Specific amino acids must be present in the peptone of the medium; otherwise these compounds will not be formed. The latter group includes the true bacterial toxins. These are of two kinds: the exotoxins and the endotoxins.
The Exotoxins:
The exotoxins are elaborated by the bacterial cells and excreted into the surrounding culture medium. These may be recovered by passing the culture through an appropriate filter to remove the bacterial bodies from the medium. Only a few pathogenic bacteria are capable of excreting true soluble toxins of great potency.
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The symptoms produced are due largely to the toxins excreted by these organisms. In other words, the injection of the cell-free filtrate produces symptoms characteristic of the disease. The best-known members of this group are Corynebacterium diphtheriae, Clostridium tetani. CI. botulinum, some of the sporuiating anaerobes isolated from gas gangrene. Streptococcus pyogenes, and Staphylococcus aureus.
The Endotoxins:
The endotoxins, on the other hand, are not excreted into the surrounding culture medium but remain confined within the bacterial cells. They are released only after the death and dissolution of the organisms. Most bacterial organisms fall in this group. An example is Salmonella typhosa, the causative agent of typhoid fever.
If a young culture of this organism is filtered, the filtrate will produce only a slight toxicity, whereas the organisms themselves may produce a very toxic effect. Filtrates of old cultures may be very toxic, owing to death and autolysis of many of the organisms, resulting in the liberation of the endotoxins.
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Some organisms are capable of elaborating both exotoxins and endotoxins. The organisms of cholera (Vibrio comma) and dysentery (Shigella dysenteriae) appear to belong to this group, although they elaborate considerably more endotoxin than exotoxin.
Resistance:
The power of the body to prevent growth and development of organisms after they have gained entrance is spoken of as resistance. The various defense mechanisms come into play and in most cases quickly remove the invading bacteria. Sometimes the resistance to a disease is characteristic of a species. It is then spoken of as immunity.
Various degrees of immunity have been shown to exist. One race may be immune to a certain disease; another may be susceptible. This does not mean that the former race cannot be given the disease. Small doses of the organisms may be easily disposed of, but massive doses are usually able to overcome the natural defenses of the host, with the result that disease develops.
Chickens are immune to anthrax because their body temperature is too high for the growth of the organism. If the body temperature is lowered to 37°C., chickens become susceptible to the disease.
Term Paper # 5. Shape and Size of Bacteria:
The bacterial cells vary greatly in their shape. All the individuals of a species have almost the same shape. True bacteria are unicellular. In some forms, a number of cells are grouped together and covered by mucilage layer.
Based on morphology, the true bacteria are divided into the following 3 main groups:
1. Spherical or Coccus Bacteria:
The ellipsoidal or spherical bacteria are called cocci (singular coccus). The cocci measure 0.5- 1.25 µm in diameter. The cocci are without flagella (atrichous), thereby non-motile. They may occur singly or in groups in different orientation.
Based on the number of cells and its arrangement in an aggregation, the cocci are of different types:
(a) Micrococci:
When the coccus occurs singly, it is called micrococcus; e.g., Micrococcus nigra, M. lutens, M. ceroliticus.
(b) Diplococci:
When cocci occur in pairs, they are called diplococci, e.g., Diplo- coccus pneumoniae.
(c) Tetracocci:
When cocci form a group of four cells, they are called tetracocci, e.g., Gaffkya tetragena, Pedicoccus cerevisiae.
(d) Staphylococci:
When spherical bacteria form an irregular group, they are called staphylococci, e.g., Staphylococcus aureus, S. albus.
(e) Streptococci:
When cocci occur in long chain, they are called streptococci, e.g., Streptococcus lactis, S. pyogenes.
(f) Sarcinae:
When spherical bacteria are arranged like a cube, made up of 8 or more cells, they are called sarcinae, e.g., Sarcina verticuli, S. lutea.
2. Rod-shaped or Bacilli Bacteria:
The bacterial cells are rod-shaped, cylindrical or rodlike, called bacilli (singular, bacillus). They may be motile or non-motile. The rods may be very short or long, narrow and have blunt or round ends.
They may occur either singly or in groups:
(a) Bacillus:
When rod-shaped bacteria occur singly, they are called bacilli, e.g., Bacillus polymyxa, B. anthracis, Lactobacillus.
(b) Diplobacillus:
When rod-shaped bacteria occur in pairs, they are called diplo- bacilli, e.g., Corynebacterium diphtheriae.
(c) Streptobacillus:
When bacilli occur in chain, they are called streptobacilli, e.g., Bacillus cereus, B. tuberculosis.
3. Spiral Bacteria:
The cells are slightly larger and spirally coiled rods, called spirilli (singular, spirillum). Each bacterium has more than one turn of a helix and one or more flagella at each pole. They occur either singly or in chain, e.g., Spirillum minus, S. volutans, S. undulum, Rhodospirillum, Microspora.
Besides the above groups, following other shapes of bacteria are also present:
A. Vibrios:
The cylindrical cell is curved and looks like the sign of comma (,) with a single flagellum at its tip, called “comma bacterium”. It measures about 10 µm x 1.5-1.7 µm, e.g., Vibrio cholerae, V. coli.
B. Filamentous:
Some bacteria are filamentlike, e.g., Beggiatoa, Thiothrix.
C. Pleomorphic:
Some are able to change their shape and size in response to variation in the surrounding environment. Acetobactor may occur as single rod (bacillus) or chain of small rods (Streptobacillus), in response to environmental variation.
Size of Bacteria:
The bacterial cells vary greatly in their size. The average diameter ranges from 0.5 µm to 2.0 µm. The size of bacteria also varies according to shape. The cocci measures 0.5 µm to 1.25 µm in diameter. The bacillus or rod shaped bacterium measures 0.5-1.0 µm x 2-3 µm.
The helical or spiral bacteria are larger in size, about 1.5 µm in diameter and up to 15 µm in length. Recently some bacteria have been identified which are much larger than the common ones. Epulopiscium fishelsohnii (200 µm x 80 µm) and Thiomargarita namibiensis (750 µm diam.) are so large that they can be visible to the naked eye.
Term Paper # 6. Classification of Bacteria:
It is beyond the scope of this book to consider in detail the classification system in use for separating the major groups of bacteria. However, it is useful to acknowledge, in general terms, how this classification is achieved.
The simplest classification is based entirely on staining characteristics (e.g. Gram-positive or Gram-negative) and morphology. However, this method alone will not differentiate significant pathogens from other organisms.
Descriptions of the colony types produced when bacteria are grown in simple, artificial media, will improve differentiation considerably in experienced hands, but this is not reliable enough for routine, diagnostic use. For this reason, a range of biochemical properties, for example, the ability to ferment certain sugars, is normally examined; the wider the range, the more accurate the designation.
In practice, a combination of all these methods is used, thereby allowing bacteria to be characterized into families, genera, species and strains. For example, a Gram-negative diplococcus (spherical bacteria in pairs), which grows aerobically on serum-enriched media, and ferments maltose and glucose, might be identified as Neisseria (genus) menintitidis (species) the causative agent of meningococcal meningitis.
Perhaps the most definitive method of classification is the examination of bacterial DNA sequence homology, although this is not a method which is routinely used in the laboratory identification of bacteria.
Routes of Acquisition:
Bacteria causing infection are acquired from two principal sources – either from amongst the patient’s own normal flora (endogenous infection) or from external sources, for example from food (exogenous infection).
Exogenous infections may be acquired by one of the four principal routes detailed below:
1. Ingestion e.g. food poisoning associated with the consumption of foods contaminated with Salmonella species.
2. Inhalation e.g. inhalation of airborne droplets containing Mycobacterium tuberculosis, leading to pulmonary tuberculosis.
3. Inoculation e.g. rose-thorn punctures introducing Clostridium tetani and leading to clinical tetanus.
4. Direct contact e.g. Neisseria gonorrhoeae, acquired by intimate person to person contact.
Term Paper # 7. Requirements for Bacterial Growth:
Bacteria display a wide range of nutritional and physical requirements for growth including:
I. Water
II. A source of energy
III. Sources of carbon, nitrogen, sulfur, phosphorus
IV. Minerals, e.g., Ca2+, Mg2+, Na+
V. Vitamins and growth factors
Microorganisms may be grown in liquid, solid or semisolid media. Liquid media are utilized for growth of large numbers of organisms or for physiological or biochemical studies and assays. Some species, such as streptococcus or Staphylococcus, often demonstrate typical morphologies only when grown in liquid media.
Solid media are useful for observations of characteristic colonies, for isolation of pure cultures and for short-term maintenance of cultures. Usually, the preparation of a solid medium for growth simply includes the addition of 1 to 2% agar to a solution of appropriate nutrients. Agar is a complex carbohydrates extracted from marine algae that solidifies below temperatures of 45°C. It is not a nutritional component.
Usually, bacteria are grown in complex media, because we simply do not know enough about the organism or organisms to define all of their requirements for growth and maintenance. Neither the chemical composition nor the concentrations of substrates are defined.
Media frequently contain nutrients in the form of extracts or enzymatic digests of meat, milk, plants or yeast. For fastidious organisms we must often use delicious-sounding concoctions such as tomato juice agar or chocolate agar, or something less appetizing (but nutrient-rich) such as brain-heart infusion broth or blood agar.
There is no single medium or set of physical condition that permits for cultivation of all bacteria, and many species are quite fastidious, requiring specific ranges of pH, osmotic strength, temperature and presence or absence of oxygen. The requirements for growth of bacteria under laboratory conditions are determined by trial and error.
You will culture bacteria using a rich, complex medium, namely tryptic soy agar or broth, so that a wide variety of possible unknowns can be mixed into the same culture and grown on the same plates.
Agar plates will be used for solution and some assays, and for short term maintenance of cultures. Agar slant tubes will be used for long term maintenance of isolates. Broths (liquid media) will be used to grow isolates for some assays or for the assays themselves.
Term Paper # 8. Importance of the Bacterial Cell Wall:
In 1884, Christian Gram observed that the majority of bacteria could be classified into two broad groups, depending upon their ability to retain crystal violet dye after decolourization. Those retaining dye were termed Gram-positive and those failing to do so Gram-negative.
This staining phenomenon, which is still of great importance in the initial laboratory identification of bacteria, results from fundamental differences in the cell walls of the two types of organism.
All bacteria are bounded by a cytoplasmic membrane, composed of a typical phospholipid bilayer, the function, of which is to supply the cell with energy via its associated enzyme systems and to regulate the passage of metabolites into and out of the cell.
Surrounding the cytoplasmic membrane is a layer of peptidoglycan, a complex polymer of polysaccharide chains linked by short peptides. This layer gives the cell its strength and shape and is much thicker in Gram-positive cells (accounting for more than 40 percent of the dry weight of the cell wall) than in Gram-negative cells (where it accounts for around 10 per cent).
In Gram positive organisms, numerous surface proteins and polymeric molecules other than peptidoglycan are also found closely associated with the peptidoglycan layer. A second outer membrane is present in Gram- negative organisms which contains lipopolysaccharide and protein molecules.
Flagella and fimbriae are cell composed of tubular filaments of polymerized protein that project from the cell wall of some Gram-negative bacterial cells. Flagella are much longer than most fimbriae and generate propulsive forces which enable the bacterium to move within a fluid medium.
Fimbriae, often also referred to as pili, are mainly involved in the adherence of bacterial cells to other bacteria and to host tissues. The notable exceptions are the sex pili which are important in the transfer of bacterial DNA, usually plasmids, from one bacterium to another.
Finally, external to the cell wall, most pathogenic bacteria, whether Gram-positive or negative, are covered with a protective layer of carbohydrate known as capsular polysaccharide.
Term Paper # 9. Transmission of Bacterial Infection:
The transmission of a bacterial infection is dependent upon several factors including the characteristics of the ‘host’ population at risk, the bacterium concerned and the nature of the environment.
Important host factors include the degree of immunity to a particular pathogen within the population, the proximity of individuals to each other and the general state of health and hygiene. It is worth mentioning here that some individuals, while apparently healthy, may harbour and transmit pathogenic bacteria—these individuals are often referred to as carriers.
For example, healthy individuals can excrete Salmonella species for prolonged period, causing outbreaks of food poisoning if they are involved in the preparation of food.
Bacterial factors include: the general properties of the organism, in particular, its virulence; its ability to survive in the environment; the size of the infecting dose; and the route by which the bacterium is acquired.
Environment factors affecting transmission include – climate (bacterial growth generally being favoured by warm humid conditions); the standard of sanitation: and the presence of non-human vectors’ for example ticks, which transmit bacteria whilst feeding on human or animal blood.
Bacteria can be transmitted between individuals of the same generation (horizontally e.g. M. tuberculosis spread by respiratory droplets) or from mother to baby (vertically). An example here is Listeria monocytogenes, which may be transmitted from mother to child in utero and cause generalized sepsis in the fetus or newborn child.
Term Paper # 10. Pathogenesis of Bacterial Diseases:
Pathogenic bacteria possess so called ‘virulence determinants’, which are responsible for their ability to cause disease. Many of these virulence determinants are cell wall constituents. An understanding of the nature and mode of action of virulence determinants is essential if we are to appreciate the mechanisms which underlay the pathogenesis of bacterial diseases.
Virulence Determinants Specific to Gram-Positive Bacteria:
Non-Peptidoglycan Polymers:
These are a heterogeneous group of teichoic acid-like polymers containing sugar alcohols and phosphodiester linkages, which are found on the surface of Gram-positive cells, bound covalently to peptidoglycan. Their precise role in the pathogenesis of disease is unclear, but they are thought to be involved in the stimulation of the inflammatory response. They are strongly immunogenic and form the identifying group antigens of many species of streptococci.
Unlike these ‘secondary’ cell wall polymers, the closely related molecule, lipoteichoic acid, lies in contact with the cytoplasmic membrane and protudes through the peptidoglycan layer. It is thought to be important in the adherence of bacteria to surfaces, in particular, the binding of decay-causing organisms, such as Streptococcus mutans, to tooth enamel.
Surface Proteins:
Many different cell surface proteins have been identified, the majority of which do not appear to be virulence factors. One notable exception, however, is the ‘M’ protein of group A betahaemolytic streptococci (e.g. Streptococcus pyogenes). By binding to various serum proteins, bacteria expressing M proteins are able to avoid recognition and ingestion by phagocytic cells and inhibit neutrophil chemotaxis.
Virulence Determinants Specific to Gram-Negative Bacteria:
Lipopolysaccharide:
Lipopolysaccharide (LPs) is one of the most important bacterial virulence factors and is often referred to as endotoxin. It is an integral part of the outer surface of the outer membrane of Gram- negative cell walls and consists of an inner glycolipid (Lipid A) attached to a ‘core’ oligosaccharide, with or without a variable length outer, ‘O’ polysaccharide.
Lipid A is a very potent toxin and is responsible for all the toxic properties attributed to endotoxin, although these are enhanced when the lipid molecule is associated with an O polysaccharide. Although incompletely understood, endotoxin exerts a profound effect when introduced into the host, producing widespread stimulation of the immune system and activation of the complement and the clotting cascades. This results in generalized damage to the host manifested in features collectively referred to as endotoxic shock, which may result in death.
The O polysaccharide chain of LPS additionally confers resistance to the bacteriolytic effects of serum and protects the bacterial cell from phagocytosis.
Outer Membrane Proteins:
Numerous protein molecules can be found within the outer bacterial membrane. They are closely associated with LPS and often difficult to purify, but do appear to have functions in cell transport systems and ion binding. In some bacterial species, however, these proteins are also major virulence factors, enabling bacterial cells to adhere to their target tissues. Particular examples are found in enteropathogenic forms of Escherichia coli (EPEC) which cause diarrhoea in young children.
In other species, such as entero-invasive E. coli (EIEC) and Shigella species, which cause a dysentery-like illness, the outer membrane proteins not only help the bacteria to adhere to the gut epithelium, but also enable them to enter the host cell where they multiply and subsequently kill the cell. The precise mechanisms of this invasive process are not yet fully understood.
Flagella and Fimbriae:
Flagellar proteins are strong immunogens and represent the ‘H’ antigens used in typing many Gram-negative bacteria, notably the Salmonellae. However, apart from conferring active motility, which may be a useful attribute in certain circumstances, it is not thought that flagella are of major importance as far as virulence is concerned.
Fimbriae, on the other hand, are very significant virulence factors. Their presence is dependent upon the conditions under which the bacteria are growing but they are often present in the majority of Gram-negative bacteria. Traditionally, fibriae have been divided into two groups, depending on whether or not their ability to agglutinate erythrocytes of several animal species can be blocked by the presence of D-mannose.
The mannose-sensitive (MS) variants are commonly encountered and are referred to as ‘common fimbriae’. They facilitate binding to a number of cells and proteins, but their precise role remains unclear.
The role of mannose-resistant (MR) fimbriae, however, is better understood, at least in certain species. The fimbriae of N. gonorrhoeae, for example, adhere to a number of host cell types. In addition, the fimbriae also prevent binding of the bacterium to leucocytes, thereby inhibiting phagocytosis.
Certain strains of E.coli isolated from patients with infections of the kidney (pyelonephritis), possess specific fimbriae that bind to glycolipids present on the lining epithelium of the upper urinary tract. Bacteria possessing such fimbriae are less likely to be flushed away by the normal flow of urine and therefore more likely to produce clinical infection.
Another example of fimbrial adherence is seen in enterotoxin-producing E. coli which causes diarrhoeal disease, including the verotoxin producing E. coli (VTEC) which can give rise to haemorrhagic colitis and renal failure. The fimbriae of these organisms adhere to the colonic epithelium allowing direct interaction between the potent toxins produced by the bacteria and the epithelial cells.
Virulence Determinants Common to Gram-Negative and Gram-Positive Bacteria:
Capsular Polysaccharides:
The polysaccharide matrix surrounding many bacteria is highly variable in structure and is often derived from either the non-peptidoglycan polymers in the case of Gram-positive organisms or the O polysaccharide chains of Gram-negative organisms, and is termed the ‘K’ antigen in enterobacteria.
Capsular polysaccharides enable the bacterium to adhere by forming a sticky layer on surfaces and are important in the formation of dental plaque and the colonization of implanted medical devices and intravenous cannulae. They also render the bacterial cell wall inaccessible to the action of complement and to phagocytosis.
Some capsular polysaccharides have the added advantage of mimicking host tissue antigens and so are not recognized as foreign by the immune system. For example, certain strains of E. coli are able to cause meningitis in newborn infants. These organisms possess the so- called K1 capsule, which is structurally similar to proteins found in the central nervous system of newborn infants. The immune system sees the K1 capsule as ‘self and the bacteria are therefore not destroyed.
Toxins and Enzymes:
Large numbers of toxins are known to be produce by bacteria. They are usually proteins of varying molecular weight and are traditionally referred to as exotoxins to differentiate them from the endotoxin of Gram-negative bacteria.
They are numerous and wide-ranging in their effects and are conveniently grouped on the basis of the following three main characteristics:
1. Site of Action of the Toxin:
Some exotoxins act only at the site at which they are released. For example, the enterotoxin of Clostridium perfringens acts locally on intestinal epithelial cells to cause diarrhoea. On the other hand, certain toxins may have more generalized systemic effects. Diptheria toxin, for example, acts systemically, inhibiting host cell protein synthesis and resulting in damage to most major organs.
2. Mode of Action:
Exotoxins may either act directly to cause their effects or their effect may be mediated through other agents. Tetanus toxin, for example, acts directly by blocking the release of neurotransmitters, leading to paralysis, whereas staphylococcal toxic shock syndrome toxin cause the release of immune mediators from macrophages, resulting in widespread tissue damage.
3. Structure of the Toxin:
The toxin of Streptococcus pyogenes, streptolysin O, is a single molecule which binds to cell membranes causing lysis, whereas diphtheria toxin, after binding to a cell, requires cleavage by proteolytic enzymes before its active component can enter the cytoplasm.
Some toxins are enzymes but many other enzymes not regarded as toxins are produced by bacteria of all types. Their role as virulence factors is unclear, although some are able to lyse molecules of immunoglobulin A (IgA), which may enable them to become more easily established on mucous membranes, while others may assist in the local spread of bacteria once infection has occurred.
Other important enzymes, which cannot be classed as true virulence factors but are nevertheless important in human disease, are the enzymes produced by bacteria to counter the effects of antibiotics used to treat infections. Examples of this are the β-lactamase enzymes produced by bacteria that are capable of inactivating penicillin-like compounds.
Factors Influencing Bacterial Virulence:
Many bacteria do not have the potential to express virulence factors and are only able to do so if they acquire the necessary genetic material from plasmids or bacteriophages. Plasmid-mediated virulence factors are important in infections caused by several Gram-negative species. As transmissible units of genetic material, plasmids offer enormous potential for the exchange and recombination of gene sequences coding for virulence.
Bacteriophages are viruses capable of infecting bacterial cells and may also mediate the transfer of genetic material from one bacterial cell to another. The best example of bacteriophage-mediated virulence is Corynebacterium diphtheriae which requires the β-phage genome in order to produce its toxin. Environmental conditions (e.g. temperature, pH, available nutrients) also influence the expression of virulence factors, although this area is still incompletely understood.