Bacterial Growth Requirements

An understanding of what and how microbes do things depends upon our ability to grow
the organisms in the laboratory. In order to culture them, we must provide them with
proper food and environmental conditions, isolate them and maintain them in
pure culture (clone).
In microbiology, the term GROWTH refers to an increase in the number of cells
(colony or culture), not an increase in the size of the individual cells.
Requirements for microbial growth are divided into two categories, physical and chemical. Physical aspects include temperature, pH, and osmotic pressure. Chemical requirements include water, sources of carbon and nitrogen, minerals, oxygen,and organic growth factors. Physical Requirements A. Temperature most microbes live within restricted ranges of temperature with a Range of Tolerance (minimum ­ maximum) which includes an OPTIMUM temperature (most rapid growth in the shortest period of time); while the optimum is usually closer to the maximum temperature, this temperature may not be best for all cellular activities psychrophiles or psychrophilic (0­-200 C) 32­-680 F mesophiles or mesophilic (25­-450 C) 77­-1130 F thermophiles or thermophilic (50­-700 C) 122­-1580 F a temperature of 750 C (1670 F) for 20 minutes will kill vegetative cells
but not endospores ­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ B. pH most bacteria grow in the range of pH near neutrality; usually between 6.5 ­-75; as bacteria grow, their metabolic activities change the pH of their living environment; i.e. the culture media; to neutralize these changes, BUFFERS are usually added to the media to counteract or neutralize these metabolic changes and to keep the pH constant; some bacteria are capable of living in what would be considered extreme pH ranges, e.g. acidophiles ­ acid lovers can be found in acid runoff with pH as low as 1.6; Thiobacillus thiooxidans; alkalophiles ­ live in basic environments such as the ocean, pH about 8.2; similar to small intestine pH vast majority of BUFFERS are PHOSPHATE compounds because they are non-toxic; they work within the range of most bacteria and can serve as essential nutrients pH Range Solution Added to Reach that pH 1.5-3.0 sulfuric acid 4.0-6.0 citric or glutamic acid 6.0-7.5 sodium or potassium phosphate 8.0-9.0 "tris" or trihydroxymethylaminomethane 10.0-11.0 sodium silicate or sodium borate ­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­ C. Oxygen ­ respiratory requirements microbes that use oxygen are called AEROBES; organisms that require oxygen in order to live are called OBLIGATE AEROBES; some representative examples are: Bacillus, Pseudomonas, Mycobacterium; microbes that CANNOT use oxygen or for whom oxygen is actively toxic are called OBLIGATE ANAEROBES; some representative examples are found in the genus Clostridium
in both of the foregoing situations, regulating the amount of oxygen present represents a means of controlling growth or the rate of growth of the microbes; e.g. subjecting dead or NECROTIC tissue to oxygen under pressure (as in a hyperbaric chamber) will force oxygen into these tissues and kill any anaerobic organism (but not its endospores) present in these areas; the most dangerous organisms are those most flexible in their oxygen requirements; organisms that can grow in the presence of oxygen OR in its absence are called FACULTATIVE ANAEROBES; representative examples include both Escherichia coli and Staphylococcus aureus Oxygen requirements fall along a continuum ranging from: OBLIGATE ANAEROBES­­­­­­­­­­­­­­­­­­­­­­­TO OBLIGATE ANAEROBES with FACULTATIVE ANAEROBES falling in the middle of this continuum. Additionally, some organisms can tolerate the presence of oxygen but not use it for growth; these are termed AEROTOLERANT ANAEROBES; other microbes can use oxygen, but only if the concentration is less than that found in the ambient air (20%); these microbes are called MICROAEROPHILIC; thus we have: OBLIGATE ­­ MICROAERO ­­ FACULTATIVE ­­ AEROTOLERANT ­­ OBLIGATE AEROBES PHILIC ANAEROBES THIOGLYCOLLATE BROTH is a liquid medium which can be used to grow all respiratory types. Chemical Requirements Microorganisms are cultured in water to which appropriate dissolved nutrients are added. These nutrients fall into three categories: 1. Energy sources 2. Cell structural components (Elemental Requirements) 3. Miscellaneous Growth factors not all kinds of organisms require the same nutrients nor can any one organism use all kinds 1. Energy Sources a. Organic energy sources ­ sugars, starches, fats, protein; glucose is the most common; acetic, glutamic, lactic acid; used by most bacteria, all fungi and protozoa b. Inorganic energy sources ­ NH4+, nitrite, iron, H2S; only bacteria use these sources (sulfur bacteria, chemoautotrophs, chemolithotrophs) c. Light ­ photoautotrophs (cyanobacterium) 2. Elemental requirements Macro­ and micro­ or trace elements or nutrients; including but not limited to: C, H, O, P, K, I, N, S, Ca, Fe, Mg 3. Miscellaneous growth factors required in small amounts and usually cannot be synthesized from other carbon sources; varies by species a. Vitamins ­ B1, biotin, pyroxidine (B6), B12, others may be needed b. Amino Acids ­ get from protein digests; e.g. casein (milk protein); peptone (meat protein) c. Purines and Pyrimidines d. Heme ­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­