Signs of life in complex organisms such as animals are self-evident, and death is made clear by loss of nervous function, respiration, or heartbeat. In contrast, death in microscopic organisms that are composed of just one or a few cells is hard to detect, because such organisms often reveal no conspicuous vital signs to begin with.
Lethal agents (such as radiation and chemicals) do not necessarily alter the overt appearance of microbial cells. Even the loss of movement in a motile microbe cannot be used to indicate death. This situation has made it necessary to develop special qualifications that define microbial death and answer the questions: How do antimicrobial agents cause death? and Why do they vary in their effects?
The destructive effects of chemical or physical agents occur at the cellular and molecular level. As a cell is exposed to an agent such as intense heat or toxic chemicals, various cell structures break down, and the entire cell can sustain irreversible damage. At present, the most practical way to detect this damage is to determine if a microbial cell can still reproduce when exposed to a suitable environment. If the microbe has sustained metabolic or structural damage to such an extent that it can no longer reproduce, even in an ideal environment, then it is considered dead. In most circumstances, microbial death is defined as the permanent loss of reproductive capacity, even under optimal growth conditions.
Factors that Affect Death Rate
The ability to define microbial death has tremendous theoretical and practical importance. It allows medicine and industry to test the conditions required to destroy microorganisms, to pinpoint the ways antimicrobial agents kill cells, and to establish standards of sterilization and disinfection in these fields. Hundreds of testing procedures have been developed for evaluating physical and chemical agents. Some examples can be found in appendix B.
The cells of a culture show marked variation in susceptibility to a given microbicidal agent. Death of the whole population is not instantaneous but begins when a certain threshold of the microbicidal agent (some combination of time and concentration) is met. Death continues in a logarithmic manner as the time of exposure is increased. Because many microbicidal agents target the cell’s metabolic processes, active cells (younger, rapidly dividing) tend to die more quickly than those that are less metabolically active (older, dormant). Eventually a point is reached at which survival of any cells is highly unlikely; this point is equivalent to sterilization.
The effectiveness of a particular agent is governed by several factors besides time. These additional factors influence the action of antimicrobial agents:
1. The number of microorganisms. A higher load of contaminants requires more time to destroy.
2. The nature of the microorganisms in the population. In most actual circumstances of disinfection and sterilization, the target population is not a single species of microbe but a mixture of bacteria, fungi, spores, and viruses, presenting a broad spectrum of microbial resistance.
3. The temperature and pH of the environment.
4. The concentration (dosage, intensity) of the agent. For example, UV radiation is most effective at 260 nm, and most disinfectants are more active at higher concentrations.
5. The mode of action of the agent. How does it kill or inhibit microorganisms?
6. The presence of solvents, interfering organic matter, and inhibitors. Saliva, blood, and feces can inhibit the actions of dis infectants and even of heat.
Some Practical Concerns in Selecting Antimicrobial Agents
A remarkable variety of substances and objects require sterilization, disinfection, and other forms of decontamination. They range from durable materials made of glass and rubber to highly sensitive liquids such as blood plasma and even human tis sues such as bone. Sometimes entire rooms with their contents and large pieces of equipment such as space vehicles and satellites may need to be sterilized. In accordance with an international agreement, all spacecraft landing on extraterrestrial bodies must have a highly reduced microbe load. This policy is to prevent the introduction of earth microbes that could potentially alter those environments or interfere with experiments being conducted by NASA. Parts of the probes are sterilized with heat, radiation, and certain chemicals (peroxide), and then maintained in very controlled clean rooms that require extreme precautions against contamination by workers (figure 1).
Fig1. Technicians in a clean room at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, examine the mirror of the James Webb space telescope. The telescope was launched on December 25, 2021. Desiree Stover/NASA
There are literally hundreds of situations in health care settings alone where prevention of infection must be taken into consideration at all times. The CDC provides guidelines for recommended sterilization and disinfection procedures.
When choosing a procedure, one of the first questions to arise is, What will render materials free of potential pathogens? Consider, for instance, reusable dental instruments that are highly contaminated with saliva, blood, and other fluids and tissues. These are all potential sources of infection, so it is critical that they be sterilized between patients with a method that will kill endospores.
The next concern is selecting the best method of sterilization. Because they are mostly made of metal, dental instruments can be sterilized with heat, although some forms of radiation and chemical treatments could also work. Clinics generally opt for a method such as steam sterilization that is simplest, fastest, and least expensive. This would not work on other types of delicate instruments such as endoscopes, which must be disinfected between patients using heat-free techniques.
On the other hand, certain single-use medical supplies used in hospitals and clinics, such as plastic syringes and catheters, come already sterilized and packaged at the factory. Because they are usually made of heat-sensitive plastics, this is done with penetrating radiation or chemicals. These items are not meant to be reused, but they still must be decontaminated before being discarded to prevent the transmission of disease. Disposal of such medical wastes is regulated by various governmental agencies.
