Unlocking Resistance: Evaluating Different Methods of Susceptibility Testing for Ceftazidime/Avibactam and Aztreonam Combination in MDR Gram-Negative Organisms

INTRODUCTION

In the current period of rising resistance to antimicrobials, the issue of multi and pan-drug-resistant organisms is a major concern. Multi-drug-resistant gram-negative organisms are often treated with last-resort drugs like polymyxins, but the increasing use of these antibiotics has led to the development of resistance. Multi-drug resistant Enterobacterales and other gram-negative bacteria cause severe infections for which no defined treatment exists. These infections can be treated by a combination of aztreonam (ATM) with ceftazidime-avibactam (CZA), but no approved testing method is available for use in clinical laboratories to assess the activity of this combination [1,2]. 

However, different testing methods such as broth disk elution, disk stacking, gradient strip stacking, and strip crossing can be used for testing the susceptibility for this combination [3].

No established treatment regimen is available for multi and pan drug resistant organisms, especially those producing metallo beta lactamases (MBL). Although last-resort antibiotics such as polymyxins are used, however their effectiveness is constrained due to their therapeutic window being narrow, which can result in toxicity and the development of resistance during treatment [4].

Klebsiella pneumoniae and Escherichia coli exhibited the highest number of sensitive strains to the combination.

This emphasizes on the in vitro efficacy of a unique combination of CAZ-AVI and ATM in multidrug resistant isolates. The kappa agreement between the strip stacking and strip cross method was excellent (value 1.000) as compared between strip methods and disk elution method (value 0.794). The results demonstrated that findings from all three methods were comparable and could be utilized for in vitro testing of the ceftazidime/avibactam and aztreonam combination.

DISCUSSION

This study assesses three different methods for determining the in vitro efficacy of the ceftazidime-avibactam and aztreonam combination. Previous researchers have tested this combination using different approaches. For instance, an Indian study by Sahu et al. also observed synergy between the disk and E-strip methods [7]. In our study, three different methods i.e., the strip stacking method, the strip cross method, and the disk elution method for ceftazidime-avibactam and aztreonam combination were compared. While the results from all three methods were comparable, the kappa agreement between the strip stacking and strip cross methods was excellent (value 1.000), in contrast to the agreement between the strip methods and disk elution method (value 0.794).

This discrepancy may be attributed to the use of similar E-strips in both the strip stacking and strip cross methods, which share a comparable methodology. In contrast, the disk elution method employed ceftazidime/avibactam and aztreonam disks instead of E-strips, which could lead to differences in efficacy or erroneous results in one of the methods. A study by Khan et al. found complete agreement between the strip stacking and strip cross methods, aligning with our findings [2]. Similarly, another Indian study recommended implementing both strip-based and disk elution methods in clinical laboratories [5].

In terms of susceptibility, higher susceptibility was observed in Klebsiella pneumoniae and Escherichia coli strains, while the highest resistance was noted in Pseudomonas aeruginosa isolates. This may be due to non-carbapenemase resistance mechanisms, such as mutations in porins or efflux pumps, which align with findings from other studies. In an international study by Avery et al., gradient strip methods were recommended in the absence of molecular tests [8].

The need for such combinations is critical for treating patients with multidrug-resistant infections. In well-established laboratories, E-strip-based methods can be easily used, although they require proper training and expertise. On the other hand, in low-resource settings, the disk elution method is more practical, as it is cost-effective, does not require specialized training, and follows a simpler methodology. The interpretation of results in the disk elution method is straightforward, based on turbidity indicating resistance, whereas the E-strip method requires the interpretation of the MIC. However, the E-strip method provides the advantage of directly offering the MIC value, with growth observable on the plates. In contrast, turbidity in the disk elution method could result from contamination, which might not be detected visually.

Thus, both methods have their respective advantages and disadvantages, and the choice of method should depend on the laboratory setting and available resources.

CONCLUSION

Ceftazidime/avibactam and aztreonam combination can be beneficial for patients infected with multidrug-resistant organisms. While there is currently no standard testing method of testing available for this combination, both E-strip and disk elution methods are viable options for in vitro antibiotic susceptibility testing.

Ethical considerations:

Approval was taken from Institutional Ethics Committee before the start of the study. (IEC/68/EX/2023) No additional invasive specimens or intervention were taken for study purpose.