Susceptibility Testing

Cultures usually require 24-48 hours before results are available. Once the culture is complete and a pure sample of the microorganism is obtained, the susceptibility testing may take about another 24-48 hours depending on the method used. There are commercial tests available that offer rapid susceptibility testing and that may produce results in less than 24 hours. Cultures for fungus and tuberculosis may take much longer — up to 6 to 8 weeks.

Yes. In certain situations, a doctor may choose a therapy while a culture is incubating and in others, he may prescribe therapy without ever ordering a culture based on knowledge and experience. While it is impossible to predict which microorganism is causing an infection unless a culture is performed, some organisms are seen more frequently than others. For instance, most urinary tract infections (UTIs) are caused by the bacterium Escherichia coli. Knowing this, a doctor may rely on current susceptibility patterns for this bacterium to choose an antibiotic that is effective in most cases. In addition, there are certain life-threatening infections that must be treated immediately, with no time to wait for the results of a culture. In other instances, a culture would not be attempted because a specimen may not be obtainable (such as with otitis media – ear infections) or the pathogen may not be easily isolated from other flora in the specimen (such as with community-acquired pneumonia). In these cases, the doctor chooses therapy to cover the most common pathogens that cause these infections.

Resistance may be innate (natural) or acquired. Natural resistance is part of the microorganism's normal physical characteristics. Since microorganisms multiply very rapidly; they go through many generations in a short period of time. There is always the potential for antimicrobial resistance to arise through a genetic change (mutation). If this change gives the microorganism a survival advantage, it may be passed on to subsequent generations.

An acquired resistance may develop through a selection process. When a patient is treated with an antimicrobial agent, the most susceptible microorganisms are the ones that are killed first. If treatment is stopped before all of the pathogens are killed, the survivors may develop a resistance to that particular antimicrobial agent. The next time they are exposed to the same drug, it may be ineffective as the bacteria and their progeny are likely to retain resistance to that antimicrobial agent.

Resistance can also develop when microorganisms that are resistant share their genetic material with susceptible ones. This may occur more frequently in a health care setting, where many patients are treated with antimicrobial agents. For instance, resistant strains of bacteria, such as MRSA (methicillin resistant Staphylococcus aureus), have been a problem in hospitals for decades and are increasingly common in the community.

When a resistance trait arises in bacteria, for whatever reason, the resistant organism may spread to other people, throughout a community, and potentially across the world. Once a strain of bacteria has become resistant to one or more antimicrobial agent, the only recourse is to try to inhibit its spread and to try to find another one that will kill it. The second or third choice antimicrobial agents that are available are often more expensive and associated with more side effects. This presents a challenge that is compounded by the fact that microorganisms are becoming resistant faster than new antimicrobial agents are being developed.

Another way to test for resistance is by using molecular methods to test for changes (mutations) in a microorganism's genetic material that enables it to grow in the presence of certain antimicrobial agents. Methicillin-resistant Staphylcoccus aureus (MRSA) contain the mecA gene that confers resistance to the antibiotics methicillin, oxacillin, nafcillin, and dicloxacillin. Detection of the mecA gene using a molecular based test allows the rapid detection of MRSA prior to culturing the bacteria. The person carrying this organism in their nasal passages can be isolated from other patients in the hospital so that the resistant staph are not transmitted to others.

Another example of testing for resistance is a test for beta-lactamase, an enzyme produced by some bacteria that makes penicillin ineffective. This test can be used to determine whether the bacteria produce this enzyme and are therefore resistant to penicillin and other similar drugs. However, this test is rarely performed and only on certain groups of bacteria, for example, Haemophilus influenzae and anaerobic bacteria.