Deep Sea Thermal Vent Bacteria—cont'd

В, Colorized micrograph of deep sea thermal vent bacterial community; the filaments of Beggiatoa are abundant.

ture and prevent water from turning to steam, it was impossible to sample the chambers and culture bacteria. Deming and Baross therefore measured protein and nu­cleic acid content at the end of the experiment, both of which appeared to increase. Based on these observa­tions they reported that bacterial growth occurred in the chambers incubated at 250° C. Their results were imme­diately questioned by many scientists. Holger Jannasch could repeatedly grow some of the bacteria from the thermal vents at temperatures of 100° to 110° C, but not at higher temperatures. No one was able to repeat the

experiments that purportedly demonstrated bacterial growth at 250° C. Independent confirmation is critical in science. Eventually it was shown by Art Yayanos at the Scripps Institute of Oceanography that the results re­ported by Deming and Baross could be explained by abiotic changes that occur at high temperature and pressure. Bacterial growth apparently had not occurred at 250° C. The initial report had not met the essential test of the scientific method—that of repeatability by others. The upper demonstrated growth temperature remains about 110° С

isms grow best at low temperatures. Such organisms, known as psychrophiles, have optimal growth tem­peratures of under 20° C. As long as liquid water is available, some psychrophilic microorganisms are ca­pable of growing below 0° C. Psychrophilic microor­ganisms are commonly found in the world's oceans and are also capable of growing in a household re­frigerator, where they are important agents of food spoilage.

Mesophiles are microorganisms that have optimal growth temperatures in the middle temperature range between 20° and 45° С Most of the bacte­ria grown in introductory microbiology laboratory courses are mesophilic. Many mesophiles have an op­timal temperature of about 37° C. Many of the normal resident microorganisms of the human body, such as Eschericia coli, are mesophiles. Similarly, most human pathogens are mesophiles and thus grow rapidly and establish an infection within the human body.

Thermophilic microorganisms are organist¹ with high optimal growth temperatures. The» philes, such as Bacillus stearothermophilus, growatm atively high temperatures, often growing only abova 40° C. The upper growth temperature for extern thermophilic microorganisms, such as those foundii deep thermal rift regions of the areas where volcaul activity heats the ocean water under very high pre; sure, is about 110° C. Water will remain in a liqiii state at temperatures above 100° С when it is such jected to high pressure. Thermophiles have optiis growth temperatures above 45° С and manyfe-mophilic microorganisms have optimal growth tef peratures of about 55° to 60° С One finds thai philic microorganisms in such exotic places as he springs and effluents from laundromats. Howe» many thermophiles can survive very low tempi tures, and viable thermophilic bacteria are route found in frozen antarctic soils.

FACTORS INFLUENCING BACTERIAL GROWTH 301

FIG. 10-13Drawing showing oxygen growth relationships for aerobic, anaerobic, and fac­ultative bacteria in tubes of nutrient media. The growth of aerotolerant anaerobes (obli-gately fermentative) would be the same as that of facultative anaerobes (organisms capable of fermentation and respiration).

Psychrophiles have optimal growth temperatures of under 20° C; mesophiles grow best between 20° and 45° C; and thermophiles grow best at higher temperatures above 45° C, with 55° to 60° С often being optimal.

Oxygen

Another factor that greatly influences bacterial growth rates is the concentration of molecular oxy­gen. Bacteria are classified as aerobes, anaerobes, fac­ultative anaerobes, or microaerophiles, based on their oxygen requirements and tolerances (FIG. 10-13). Aerobic bacteria (obligate aerobes)grow only when oxygen is available to support their respiratory metabolism. In laboratory cultures and industrial batch cultures, oxygen is often supplied by forced aeration or mixing (for example, on a rotary shaker) to support the growth of aerobes. Anaerobic bacteria(obligate anaerobes)grow in the absence of molecu­lar oxygen. Anaerobic bacteria may carry out fer­mentation or anaerobic respiration to generate ATP. Some anaerobes have very high death rates in the presence of oxygen, and such organisms are termed strict anaerobes. Even the briefest exposure to air can kill strict anaerobes. Other obligately anaerobic bac­teria, although unable to grow, have low death rates in the presence of oxygen.

While obligate anaerobes grow only in the absence of molecular oxygen, facultative anaerobessuch as I coli can grow with or without oxygen. Many facul­tative anaerobes are capable of both fermentative and respiratory metabolism. Some are capable of both aerobic and anaerobic respiration.

Aerobes need oxygen to support their respiratory metabolism and anaerobes grow in the absence of molecular oxygen, carrying out fermentation or anaerobic respiration; facultative anaerobes grow aerobically or anaerobically.

Although oxygen is required for the growth of many microorganisms, it can also be toxic. Some mi­croorganisms grow only over a very narrow range of oxygen concentrations. Such microorganisms are known as microaerophiles.Microaerophiles require oxygen but exhibit maximal growth rates at reduced oxygen concentrations because higher oxygen con­centrations are toxic to these organisms.

Oxygen can exist in several energetic states, some of which are more toxic than others. One of these en­ergetic states, called singlet oxygen, is a chemically reactive form that is extremely toxic to living organ­isms. Phospholipids in bacterial plasma membranes can be oxidized by singlet oxygen, leading to a dis­ruption of membrane function and the death of bac­terial cells. Peroxidases in saliva and phagocyte cells (blood cells involved in the defense mechanism of the human body against invading microorganisms) generate singlet oxygen, accounting in part for the antibacterial activity of saliva and the ability of phagocytic blood cells to kill invading microorgan­isms.

Singlet oxygen is chemically reactive and extremely toxic to living organisms.

The conversion of oxygen to water occurs when oxygen serves as a terminal electron acceptor in res­piration pathways. This involves the formation of an

METHODOLOGY

Date: 2015-02-28; view: 1307