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Antibiotic susceptibility testing – fighting AMR through diagnostics

In the case of a bacterial infection, antibiotic susceptibility testing (AST) is performed in order to identify which treatment regimen is specifically effective for individual infections [1]. When it is clear which species of bacteria is causing the infection, it can then be treated accordingly. In order to do this, it is necessary to know which antibiotic and which dosage will be effective. Therefore, AST information helps clinicians in choosing the right diagnostics-based treatment for patients.

The correct identification of a bacterial infection combined with susceptibility testing can make a significant impact on the growing rate of antimicrobial resistance (AMR). Inappropriate diagnosis and its uncertainties are one of the key drivers of antimicrobial overuse and misuse. When suitable tests are ordered and rapid diagnostic results available, they provide guidance on created tailored antimicrobial treatments to optimise patient health outcomes. This is not only the most beneficial pathway for the patient, who is getting targeted therapy to get them on the road to recovery quicker, but it is also saving the health care system unnecessary costs.

It is often reported that investing into the production of new antimicrobials is not attractive for pharmaceutical companies because there is little return on investment [2]. This urgently needs to be addressed, with sustainable funding a top priority, but it is also one big reason why focusing on diagnostic tests in hospitals and outpatient settings is vital – it’s something we can realistically improve if the right awareness and stewardship is in place. AST could support the continued use of common antibiotic drugs and spare antibiotics which are reserved for patients who have developed a high resistance against first-line treatment.    

Why should antibiotic susceptibility testing be performed?

Treatment of infections should be targeted, but that’s often not the case in practice. For example, up to 80% of suspected urinary tract infections turn out to be negative and not need antibiotics at all [3]. One of the strongest drivers of antimicrobial resistance is the overuse and misuse of antibiotics, especially in countries where antibiotics are sold over the counter.

Knowing the resistance profile of the bacterial infection will help determine the right antibiotic needed to treat it, and an antibiotic susceptibility test is the only alternative to know which antibiotic will be effective. Using targeted therapy will knock out infections quicker and more easily, giving less room for antimicrobial resistance to grow stronger [4].


Urinary tract infections and antibiotic susceptibility testing

To screen for a urinary tract infection (UTI), the dipstick is the most frequently used method. This method picks up the presence of nitrite, a product derived by certain nitrogenic species (e.g., Escherichia, Proteus, Klebsiella). However, some pathogens do not generate nitrite (e.g., Enterococcus, Gonococcus, Staphylococcus, Pseudomonas), meaning that nitrite is not always a reliable parameter to screen a suspected UTI. Additionally, a high white blood cell count does not always indicate a bacterial infection. Therefore, if typical symptoms are present, a negative dipstick result is insufficient to rule out an infection [5].

Although it may not be time efficient or cost-effective, urine culture remains a very important test in the context of UTI diagnostics, particularly for isolating the infectious microorganism. From a positive urine culture, species identification and antibiotic susceptibility testing can determine the sensitivity and resistance of specific pathogens to a range of antibiotics so healthcare providers can prescribe the proper treatment [1]. This is not yet standard practice across the globe, but making this transition could have a huge impact on the fight against AMR.

Being able to quickly exclude urinary tract infections (UTIs) could help to reduce the number of antibiotics being prescribed unnecessarily. Therefore, a fast and reliable method to exclude UTIs can be a driver for laboratory workflow improvements and antimicrobial stewardship.

What you need to know about UTI

Urinary tract infections are common, but the diagnosis and treatment process could still use some improvement. Discover how Sysmex can contribute to simplifying UTI testing.

Traditional vs. newer AST techniques

Antibiotic susceptibility testing is certainly not a new concept and there are several methods that have been used over the years. The disk diffusion susceptibility method (also referred to as the Kirby-Bauer method) is considered the gold standard for confirming the susceptibility of bacteria, and even the general public may be familiar with images of little disks in an agar plate. In this method, a bacterial inoculum is applied to an agar plate and paper disks are then placed on the surface, impregnated with different types of antibiotics. The plates are incubated for 16-24 hours and the results are determined by the diameter of the growth inhibition around each antibiotic disk [6]. This technique is simple and affordable, but it is labour-intensive, and results can take several days to reach the patient.

One of the earliest AST methods was the microbroth or tube-dilation method, which involves preparing dilutions of antibiotics in a liquid growth medium dispensed in test tubes, inoculating them with a standardised bacterial suspension, incubating them overnight and then examining for visible bacterial growth. It was not considered the most precise method due to the manual nature of preparing the dilutions of antibiotics manually. It was also a tedious task, as well as requiring a lot of space and resources [6]. Tools that provide automation for this process are available, however they are not suited for rapid or direct AST.

It became clear at quite an early stage that many of the AST techniques were quite variable and that a standardised method would be necessary to make progress. Current methods are standardised and sometimes automated, but they rely on phenotypically testing bacteria, wherein results are often not available until two days after testing and/or treatment has begun [7]. Shortening this time of testing to result, while keeping the phenotypic aspect of the test, could have a huge impact in the healthcare sphere in general, and especially when it comes to the growing rates of antimicrobial resistance.

How to perform real-time Antimicrobial Susceptibility Testing (AST)

Diagnostics play a major role in the rational prescription of antibiotics. Well-known antimicrobial susceptibility tests (AST) are reliable tools for the phenotypic determination of resistance to antibiotics [6]. Several solutions to support AST performance are available on the market.

Different methods of AST are utilised depending on the suspected strain of bacteria and what kind of resources are available, but there are also newer technologies under development to improve and speed up the AST process. Microfluidics-based diagnostics are one of the most promising emerging tools for AST [8]. A sample is loaded onto a microfluidic chip and any bacteria present are caught in bacteria-sized traps. The trapped bacteria are monitored, whereby the loading time gives an estimate of the density of bacteria in the sample. Bacterial growth is monitored in each trap, some of which are exposed to a candidate antibiotic. The average growth rates are calculated in real time and the bacteria are considered susceptible if their growth is prevented [9]. Microfluidic devices coupled with optical detection can perform AST and reduce the time to results from hours to minutes [8], which could be a huge step in the right direction when it comes to fighting the overuse and misuse of antimicrobials.

Antimicrobial susceptibility testing for near-patient settings

PA-100 AST System

PA-100 AST System

Antimicrobial susceptibility testing for near-patient settings


Trends in AMR

Addressing how we prescribe and treat with antibiotics is certainly not the only way we should be addressing the fight against antimicrobial resistance, but it is a large part of it. The availability and marketing of antibiotics differs significantly between countries, which is an important factor to be aware of when setting national guidelines [10]. A 2019 study on UTI treatment guidelines across Europe found that fluoroquinolones, a class of antibiotics often used as broad-spectrum treatment in bacterial infections, have a high variation of resistance in E. coli anywhere from 11% to 32.8%.

What the future of antimicrobial stewardship could look like

When it comes to growing resistances on a global scale, we’re not only talking about antibiotic-resistant strains of bacteria. We’re also considering viruses, fungi and parasites becoming resistant to antivirals, antifungals and antiparasitics, respectively. Early and accurate diagnosis of is a critical aspect of efforts to control the rate of AMR. Sysmex currently has solutions that can support laboratories and clinics in detecting bacteria, based on which clinicians can identify infections and evaluate treatment success (e.g. UF-5000).

Nowadays health care workers and patients become more and more aware that inappropriate treatment with antibiotics causes many other problems in addition to AMR. This leads some countries to guide the health care system to a more rigid prescription scheme. But this can still lead to inappropriate treatment and may cause even more severe disease in patients who are suffering from a bacterial infection. Therefore, more evidence-based diagnostics and treatment is required.

We are always working on improving the status quo and investing time and resources in promising new methods and technologies. New diagnostic methods and technologies are among the main pillars of stewardship programmes, strengthening the need for well-informed stewards in our governments, as well as stewards in our hospitals and doctor’s offices at the point of care, and in our everyday lives [11]. If there are widespread awareness campaigns to educate the public on the topic of antibiotic susceptibility testing and antimicrobial resistance, this would be a great start.

If we can make policymakers more aware that the growing threat of AMR is one that we can already address, we may have more power to not only enable more wide-reaching awareness campaigns, but also get resources for the necessary tools in hospital and point-of-care settings. Point-of-care tests could bring the diagnostics into outpatient clinics and thus facilitate evidence-based medication in places where extensive prescription of antibiotics happens [12]. 

Antimicrobial resistance has been around for a while, and it doesn’t seem to be going anywhere any time soon. Therefore, we must develop new solutions on multiple levels, from pharmaceutical innovation, to rapid and reliable diagnostics for various health care settings, to awareness among the public.

Antimicrobial resistance #AMRfighter

We are passionate about informing on the topic of AMR and doing what we can in the diagnostics industry. Test your knowledge of AMR here.


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[2] Klug DM et al. (2021): There is no market for new antibiotics: this allows an open approach to research and development. Wellcome Open Res. 2021 Jun 11;6:146. doi: 10.12688/wellcomeopenres.16847.1. PMID: 34250265; PMCID: PMC8237369.

[3] Keller P. (2019): Ein neuer Schritt zur schnelleren Urinanalytik. Sysmex xtra 02/2019:50-52.

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[7] Wheat PF. (2001): History and development of antimicrobial susceptibility testing methodology. Journal of Antimicrobial Chemotherapy, 48(suppl_1), 1-4.

[8] Khan ZA, Siddiqui MF, Park S. (2019): Current and Emerging Methods of Antibiotic Susceptibility Testing. Diagnostics. 9(2):49.

[9] Baltekin Ö et al. (2017): Antibiotic susceptibility testing in less than 30 min using direct single-cell imaging. Proceedings of the National Academy of Sciences, 114(34), 9170-9175.

[10] Malmros K et al. (2019): Comparison of antibiotic treatment guidelines for urinary tract infections in 15 European countries: results of an online survey. International journal of antimicrobial agents. 2019 Oct 1;54(4):478-86.

[11] CDC. (2019): Core Elements of Hospital Antibiotic Stewardship Programs. US Department of Health and Human Services, CDC

[12] Vasala A et al. (2020): Modern tools for rapid diagnostics of antimicrobial resistance. Frontiers in Cellular and Infection Microbiology, 10, 308.

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