PHYSICAL FACTORS THAT AFFECT MICROBIAL GROWTH
Generally,an increase in temperature will increase enzyme activity. But if temperatures get too high, enzyme activity will diminish and the protein (the enzyme) will denature.
On the other hand, lowering temperature will decrease enzyme activity. At freezing temperatures enzyme activity can stop. Repeated cycles of freezing and thawing can denature proteins. In addition, freezing causes water to expand and also forms ice crystals, hence cells begin to rupture.
Every bacterial species has specific growth temperature requirements which is largely determined by the temperature requirements of its enzymes. Each organism will have:
- a MINIMUM GROWTH TEMPERATURE,
- an OPTIMUM GROWTH TEMPERATURE,
- and a MAXIMUM GROWTH TEMPERATURE.
Organisms can be classified according to their optimum growth temperature.:
- PSYCHROPHILES grow best between -5oC and 20oC,
- MESOPHILES grow best between 20oC and 45oC and
- THERMOPHILES grow best at temperatures above 45oC.
- THERMODURIC organisms can survive high temperatures but don't grow well at such temperatures. Organisms which form endospores would be considered thermoduric.
Some organisms have exotic temperature requirements. Thermus aquaticus is a bright orange gram negative rod isolated from hot water and steam vents at Yellowstone Park. This organism grows best at temperatures between 70-75oC (158-167oF). Some of its unique enzymes are in demand for molecular biological and industrial applications.
Microbes display a great diversity in their ability to use and to tolerate oxygen. In part this is because of the paradoxical nature of oxygen which can be both toxic and essential to life.
2O2- + 2H+ ----> H2O2 + O2
and catalase catalyzes:
2H2O2 ---> 2H2O + O2
Oxygen levels can be manipulated in the laboratory in several ways:
- 1. The CO2 jar or CANDLE JAR lowers O2 levels below 10% (from 20%) and raises CO2 levels from 0.03% to about 10%. In addition to lowered oxygen levels, some organisms flourish better in an enriched CO2 environment. Such organisms are called CAPNOPHILIC.
- 2. The ANAEROBE JAR uses hydrogen gas and a palladium catalyst to convert oxygen into water. An indicator strip impregnated with methylene blue is used to indicate when reducing conditions (anaerobic conditions) have been achieved. When reduced, methylene blue is colorless, when oxidized this dye is blue.
- 3. THIOGLYCOLLATE is a chemical reducing agent often put into broth media to eliminate free oxygen.
Representative Anaerobic Pathogens:
1. Clostridium tetani - agent of tetanus, puncture wounds, produces a toxin which enters the spinal column and blocks the inhibitory spinal motor neurons. This produces generalized muscle spasms or spastic paralysis. The muscle of the jaw are often the first affected, hence the name LOCKJAW.
2. Clostridium botulinum - this soil organism is the causative agent of botulism which typically occurs after eating home canned alkaline vegetables which were not heated enough during canning. The neurotoxin blocks transmission across neuromuscular junctions and this results in flaccid paralysis.
3. Clostridium perfringes and Clostridium sporogenes - these organisms are associated with invasive infections known as GAS GANGRENE.
4. Clostridium difficile - the causative agent of pseudomembranous colitis, a side effect of antibiotic treatment which eliminates the normal flora.
The Streptococci - Microaerophiles
These organisms are all catalase negative, therefore the catalase test is useful in identification. They also have distinctive colonial morphology on blood agar which is differential for them. It is important to note if the colonies are alpha, beta, or gamma hemolytic.
1. Group A Streptococcus - Streptococcus pyogenes
This beta hemolytic organism is also bacitracin sensitive. It is the cause of strep throat, rheumatic fever, glomerulonephritis and scarlet fever.
2. Group D Streptococcus - Enterococcus - Streptococcus faecalis
This organism is a normal inhabitant of the large intestine. It is also a frequent cause of bladdder infections
3. Streptococcus pneumoniae.
This organism is a normal inhabitant of the repiratory tract. It is a frequent cause of pneumonia in people who have been compromised by other illness.