Nutrition
In terms of carbon utilization, bacteria may be either herterotrophic or autotrophic. Heterotrophs obtain their carbon source from organic compounds such as amino acids and sugars. Most heterotrophic bacteria are saprophytes, meaning that they obtain their nourishment from dead organic matter. Autotrophs use carbon dioxide as their sole source of carbon for growth and obtain their energy from light (photoautotrophs) or from the oxidation of inorganic compounds (chemoautotrophs).
Growth Requirements
Temperature
All bacteria have an optimal growth temperature range. Those that grow best between 20 and 40°C are referred to as mesophiles and include all human pathogens. Bacteria with higher and lower optimum temperatures are called thermophiles and psychrophiles respectively.
pH
Most bacteria grow optimally at pH values close to neutrality (pH 7). For this reason, body fluids provide a good growth environment for bacteria. However, certain bacteria prefer acid (acidophiles) or alkaline conditions (alkalophiles).
Oxygen
Whether or not a particular organism can exist in the presence of oxygen depends upon the distribution of certain enzymes such as superoxide dismutase and catalase. These enzymes deal with free oxide radicals and hydrogen peroxide, highly reactive substances which cause the disruption of hydrogen bonds in proteins. Bacteria are classified into the following four groups according to their specific requirements regarding oxygen.
-
Obligate aerobes only grow in the presence of oxygen and are unable to carry out fermentation.
-
Obligate anaerobes only grow in the absence of oxygen.
-
Faculative aerobes normally grow anaerobically but can manage to grow aerobically.
-
Faculative anaerobes normally grow aerobically but are able to grow in the absence of oxygen.
Nutrients
These include sources of organic carbon, nitrogen, phosphorus, sulphur and metal ions. All bacteria require inorganic ions in order to function including magnesium for cell wall stability, manganese and nickel for metabolic enzymes and iron for cytochromes (energy metabolism), and certain enzymes. Although high concentrations of ions usually inhibit growth, some bacteria, halophiles, grow only in the presence of high concentrations of certain salts. For example, Halobacteriaceae grow only in the presence of 3-4 M NaCl. This amount of salt is needed to maintain the structure of the cell wall and internal molecular assemblies such as ribosomes.
Bacterial Reproduction
Most bacteria reproduce by a relatively simple asexual process called binary fission: each cell increases in size and divides into two cells. During this process there is an orderly increase in cellular structures and components, replication of the bacterial DNA, and formation of a septum or cross wall which divides the cell into two. The time interval required for a bacterial cell to divide or for a population of cells to double is called the generation time. Generation times for bacterial species growing in nature may be as short as 15 minutes or as long as several days.
Growth Cycle
Populations of bacteria grown in liquid media with limited amount of nutrients, show the following characteristic growth curve.
Four characteristic phases of the growth cycle are recognized.
-
Lag Phase. Immediately after inoculation of the bacteria into fresh medium, the population remains temporarily unchanged. Although the number of cells is not increasing, the cells may be growing, synthesizing enzymes, proteins or RNA, and increasing in metabolic activity. The length of the lag phase is dependent on a wide variety of factors including the size of the inoculum, the time necessary to adapt to the new environment and the time required for the synthesis of essential enzymes or division factors.
-
Exponential (log) Phase. The exponential phase of growth is where all the cells are dividing regularly by binary fission. At this phase there is no factor limiting the growth of the bacteria. The rate of exponential growth of a bacterial culture is expressed as generation time.
-
Stationary Phase. The rate of increase is not maintained and a point is reached where the death rate equals the birth rate. Population growth is limited by one of the following factors:
- exhaustion of available nutrients
- low oxygen content
- lack of space
- change in pH
Bacteria that produce secondary metabolites, such as antibiotics, do so during the stationary phase of the growth cycle.
-
Death Phase. If incubation continues after the population reaches stationary phase, a death phase follows, in which the viable cell population decreases. At this point all of the food and nutrients have been used up and toxic secondary metabolites begin to kill cells.
Genetic Exchange in Bacteria
Although bacteria do not undergo sexual reproduction, they are able to exchange genes and undergo genetic recombination. Bacteria are known to exchange genes in nature by three processes: conjugation, transduction and transformation. Conjugation involves cell-to-cell contact as DNA crosses a sex pilus from donor to recipient. During transduction, a virus transfers the genes between mating bacteria. In transformation, DNA is acquired directly from the environment, having been released from another cell. Genetic recombination can follow the transfer of DNA from one cell to another leading to the emergence of a new genotype (recombinant). It is common for DNA to be transferred as plasmids between mating bacteria. Since bacteria usually develop their genes for drug resistance on plasmids (called resistance transfer factors, or RTFs), they are able to spread drug resistance to other strains and species during genetic exchange processes.
Bacterial Pathogenicity
Some bacteria are parasites of plants or animals, meaning that they grow at the expense of their eukaryotic host and may damage, harm, or even kill it in the process. Such bacteria that cause disease in plants or animals are pathogens. Human diseases caused by bacterial pathogens include tuberculosis, diphtheria, tetanus, syphilis, pneumonia, cholera and typhoid fever. The bacteria that cause these diseases have special structural or biochemical properties that determine their virulence or pathogenicity. These include: (1) ability to colonize and invade their host; (2) ability to resist or withstand the antibacterial defences of the host; (3) ability to produce various toxic substances that damage the host. Plant diseases, likewise, may be caused by bacterial pathogens. More than 200 species of bacteria are associated with plant diseases.
Industry and Biotechnology
In addition to other ecological roles bacteria are used industrially in the manufacture of foods, drugs, vaccines, insecticides, enzymes, hormones and other useful biological products. In the food industry lactic acid bacteria such as Lactobacillus and Streptococcus are used in the manufacture of dairy products such as yoghurt and cheese. In the pharmaceutical industry, vaccines against diphtheria, cholera and tetanus are made from components of the bacteria that cause the respective diseases.
Control of Microbial Growth
Antiseptics are chemicals used to inhibit (cidal agents) or prevent the growth (static agents) of microorganisms on living tissue. Joseph Lister first introduced aseptic surgery in 1867 when he used a spray of carbolic acid (phenol) to prevent infection. Chloroxylenol, the main constituent of Dettol, is an aromatic compound derived from phenol and is used as both an antiseptic and disinfectant.
Bacterial Resistance to Disinfectants
Resistance is a decrease in susceptibility or a lack of susceptibility of a microbe toward an agent.
As disinfectants are non-selective in their attack, they kill bacteria on contact. As a result the development of bacterial resistance is much more difficult than antibiotic resistance and would not normally be the result of a single mutation. The only way microbes could not be killed after coming into contact with the disinfectant is if they could somehow protect their cell membrane from becoming permeated.
The following mechanisms of resistance have developed:
Formation of Biofilm (Extrinsic Resistance)
Microorganisms have evolved with specialized structures aiding in their survival. Certain bacteria produce a gelatinous material known as exopoly-saccharide to form a biofilm. Biofilms help organisms stick to environmental surfaces and physically protect them from the penetration of disinfectants or other detrimental environmental conditions. While the bacteria that initiate biofilms are often not harmful, pathogenic bacteria may also stick to the biofilm and share the protective nature of the environment.
Production of Spores (Intrinsic Resistance)
Vegetative cells become spores when they’re deprived of essential growth nutrients. Formed under conditions of stress and in Gram positive strains, bacterial spores can survive extreme heat, cold, drying and chemical exposures.