Antimicrobial Susceptibility Testing for Effectiveness of Antibiotics

Antimicrobial Susceptibility Testing 

Antimicrobial susceptibility testing, also known as antibiotic susceptibility testing, is a laboratory procedure that determines which antibiotics will effectively treat a bacterial infection. The test helps healthcare professionals choose the most appropriate antibiotic when a patient is diagnosed with a bacterial infection. It prevents prescribing antibiotics that the bacteria have become resistant to.

How Antimicrobial Susceptibility Testing Works

In Antimicrobial Susceptibility Testing , a sample of the bacteria causing an infection is taken from the patient. This can be a fluid sample like blood, urine or sputum, or a tissue sample. The isolated bacteria are then incubated with different antibiotics to see how susceptible or resistant they are.

The antibiotics used in the test are determined based on the type of bacteria isolated and the most common antibiotics used to treat infections caused by that bacterium. Standardized concentrations of antibiotics are placed onto culture plates inoculated with the bacteria.

The plates are then incubated at body temperature for 16–24 hours. During this time, the bacteria multiply. Susceptible bacteria will be inhibited from growing in the area containing the antibiotic concentration, creating a clearzone around the antibiotic. Resistant bacteria will not be inhibited and will grow in the presence of the antibiotic.

By measuring the size of the clear zone, the laboratory can determine if the bacteria are susceptible, intermediate or resistant to each antibiotic tested. This provides clinicians with guidance on which antibiotics may effectively treat the patient's infection. The process takes 1-3 days to complete from sample collection.

Factors that Impact Susceptibility Testing Accuracy

Several factors can affect the accuracy and reproducibility of antimicrobial susceptibility testing results. Proper laboratory technique is crucial, such as using standardized inoculum concentrations of bacteria and antibiotic concentrations. Variations in procedure or reagents between labs can produce differing results for the same bacteria and antibiotic combination.

The health status of the individual patient also impacts results. Immunocompromised patients like those with AIDS or undergoing chemotherapy may have higher minimum inhibitory concentrations for certain antibiotics due to a weaker immune system. test results for these patients require careful clinical interpretation.

Bacteria constantly evolve and change over time as well. Resistance genes can spread between bacterial species on plasmids. This makes previous susceptibility results only useful for a limited period of around 2 years, requiring re-testing for accurate guidance on treatment.

Evolving Resistance Patterns Pose Challenges

One of the biggest ongoing challenges with antimicrobial susceptibility testing is evolving antibiotic resistance in bacteria. As antibiotics are widely used in medicine and agriculture, they place evolutionary pressure on bacteria to develop resistance mechanisms. This reduces the number of effective treatment options identified by susceptibility testing over time.

Globalization and international travel have accelerated the worldwide spread of multidrug-resistant "superbugs" like MRSA and CRE. Underappropriate antibiotic use in humans and agriculture provides further selection pressure for resistance. It takes an estimated 20 years for new antibiotics to be developed on average, but resistance can emerge much faster.

To stay ahead of resistance trends, public health agencies worldwide monitor changing susceptibility patterns in common pathogens. Surveillance informs treatment guidelines as new resistances emerge, helping optimize antibiotic use. Source control for resistant outbreaks also aims to limit their spread. However, inherent challenges mean resistance will likely continue complicating therapy for the foreseeable future.

The Future of Antimicrobial Susceptibility Testing

New technologies are revolutionizing antimicrobial susceptibility testing. Molecular diagnostic assays can detect resistance genes directly in just a few hours compared to overnight culture. They provide results more rapidly to guide initial antibiotic therapy in serious infections.

Whole genome sequencing continues decreasing in cost and allows determination of an organism's full resistance profile rather than individual drugs. Combined with clinical data, this enhanced understanding of multidrug-resistant strains will improve infection control strategies.

In the future, rapid clinical diagnostics incorporating susceptibility results promise to optimize antibiotic selection from the beginning. Alongside global action against inappropriate use, these advances aim to prolong the effectiveness of existing treatments by slowing development of new resistance mechanisms. With continued innovation, susceptibility testing remains essential for evidence-based antimicrobial therapy now and in years to come.

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