Antimicrobial susceptibility testing of Enterobacteriaceae: determination of disk content and Kirby-Bauer breakpoint for ceftazidime/avibactam

Background Detection of ceftazidime/avibactam (CAZ/AVI) antibacterial activity is absolutely vital with the rapid growth of carbapenem resistant Enterobacteriaceae (CRE). But now, there is no available automated antimicrobial susceptibility testing card for CAZ/AVI, so Kirby-Bauer has become an economical and practical method for detecting CAZ/AVI antibacterial activity against Enterobacteriaceae. Result In this study, antimicrobial susceptibility testing of CAZ/AVI against 386 Enterobacteriaceae (188 Klebsiella pneumoniae, 122 Escherichia coli, 76 Enterobacter cloacae) isolated from clinical patients was performed by broth microdilution. Of the 386 strains, 54 extended spectrum β lactamases negative (ESBL(−)), 104 extended spectrum β lactamases positive (ESBL(+)), 228 CRE. 287 isolates were susceptible to CAZ/AVI and 99 isolates were resistant to CAZ/AVI. At the same time, to obtain optimal content avibactam (AVI) disk containing ceftazidime (30 μg), inhibition zone diameter of four kinds of ceftazidime (30 μg) disk containing different AVI content (0 μg, 10 μg, 25 μg, 50 μg) were tested by Kirby-Bauer method. The microdilution broth method interpretation was used as the standard to estimate susceptible or resistance and then coherence analysis was carried out between Kirby-Bauer and broth microdilution. The result shows the inhibition zone diameter of 30 μg/50 μg disk, susceptible isolates: 20.5 mm–31.5 mm, resistance isolates: 8.25 mm–21.5 mm. The inhibition zone diameter of 30 μg/25 μg disk, susceptible isolates: 19.7 mm–31.3 mm, resistance isolates: 6.5 mm–19.2 mm. The inhibition zone diameter of 30 μg/10 μg disk, susceptible isolates: 19.5 mm–31 mm, resistance isolates: 6.5 mm–11 mm. The inhibition zone diameter of ceftazidime (30 μg), susceptible isolates: 6.5 mm–27.5 mm, resistance isolates 6.5 mm. Conclusion Our results show that 30 μg/50 μg, 30 μg/25 μg, 30 μg/10 μg CAZ/AVI disk have significant statistical differences to determinate CAZ/AVI antibacterial activity, but for 30 μg/50 μg disk, there has a cross section between susceptible isolates (minimum 20.5 mm) and resistance isolates (maximum 21.5 mm). For 30 μg/25 μg disk, it is hard to distinguish the difference between susceptible isolates (minimum 19.7 mm) and resistance isolates (maximum 19.2 mm), so 30 μg/10 μg CAZ/AVI disk is more conducive to determinate antibacterial activity.


Discussion
Increasing carbapenem-resistant Enterobacteriaceae (CRE) have drawn great attention because of their broad resistance spectra and outbreak epidemics [11,12], as well as have shown a high potential of rapid disseminations [13]. In 2012, a study reported that 3% of ICU patients in Chicago were infected with CRE, among ICU medical staff, the colonization rate of CRE was as high as 30% [14]. In 2016, seven children were infected with non-clonal CRE in pediatric hospital in Mexico [15]. Between 2005 and 2010, carbapenem-resistant Klebsiella pneumoniae invasive infections occurred successively in Greece, Italy, Hungary and Cyprus [16]. European infection control experts reported the interregional transmission or epidemic of CRE in 13/38 countries in 2015, compared with in 6/38 countries in 2013 [17]. Between 2004 and 2013, CRE has gradually increased in medical centers and major medical teaching centers, and prevalence rates of CRE isolated from ICU rose from 3.7% in 2008 to 15.3% in 2017 in Taiwan [18]. In addition, in ICU, the isolation rate of CRE Escherichia coli increased from 1.2% in 2008 to 4.0% in 2017 in medical centers, while the CRE isolation rate in major teaching centers increased from 1.0% in 2008 to 2.8% in 2017 [18]. Because of high mortality (> 30%), infection caused by CRE has become the major worrying health event all around the world [19][20][21]. In 2017, the World Health Organization listed CRE as the first "critical priority pathogens" [22]. These scattered findings in different parts of the world emphasize the fact that the development of new antibiotics against CRE is imminent. Currently, CRE treatment mainly depends on older agents, such as polymyxins, fosfomycin, tigecycline and aminoglycosides, which have been rarely used due to efficacy and/or toxicity concerns [23]. Recently, several drugs were tested to gauge their effectiveness against CRE infections, such as ceftazidime/avibactam (CAZ/AVI), CAZ/AVI is a fixed-dose combination drug containing an antibiotic-third generation cephalosporin ceftazidime and a novel non-β-lactam β-lactamase inhibitor avibactam [24,25]. Previously approved β-lactamase inhibitors such as tazobactam and clavulanic acid do not inhibit important classes of β-lactamases, including Klebsiella pneumoniae carbapenemases (KPCs), New Delhi metalloβ-lactamase 1 (NDM-1), and AmpC-type β-lactamases [26]. Avibactam can inhibit KPCs, AmpC, and some Class D β-lactamases [25]. Therefore, CAZ/AVI has preferable antibacterial activity against the majority CRE.  OXA-2 duplication [30], and loop deletion in AmpC beta-lactamase [31]. In addition, mutations in membrane porin [29,32,33], enhanced efflux pump activity [29], and sustained high expression of certain beta lactamases can lead to CAZ/AVI resistance [32]. So in the followup study, we will explore the specific resistance mechanisms. Here, our data shows resistant percentages of Klebsiella pneumonia, Escherichia coli, and Enterobacter cloacae are 20.3% (27/133), 70.7% (41/58), and 83.8% (31/37) for CAZ/AVI, respectively, which inconsistent with other researchers [34][35][36]. The follow two causes may be account for the difference. Firstly, our isolates collected according to random selection within group (ESBL negative isolates collected from ESBL negative group, ESBL positive isolates collected from ESBL positive group, CRE isolates collected from CRE group). Second, the last 2 years, new CRE organisms increasing rapidly, such as NDM-1-producing Enterobacteriaceae, which can not be inhibited by AVI [37]. Although CAZ/AVI had a good antibacterial activity against multidrug-resistant Gram-negative bacteria, it was not until 2017 that there was a cut off CAZ/AVI against Enterobacteriaceae in the report of European Committee on Antimicrobial Susceptibility Testing (EUCAST, document. 2017). In 2017, EUCAST report CAZ/AVI inhibition zone diameter cut off and MIC breakpoint, for Kirby-Bauer method (disk content 10 μg /4 μg), susceptible (inhibition zone diameter >13 mm) and resistant (inhibition zone diameter <13 mm), for microdilution broth method (a fixed concentration 4 μg/ml), susceptible (MIC<8/4 μg /ml) and resistant (MIC>8/4 μg /ml). CLSI reported CAZ/AVI inhibition zone diameter cut off and MIC breakpoint in 2018 edition, for Kirby-Bauer method (disk content 30 μg /20 μg), susceptible (inhibition zone diameter ≥ 21 mm) and resistant (inhibition zone diameter ≤ 20 mm), for microdilution broth method (a fixed concentration 4 μg /ml), susceptible (MIC≤8/4 μg /ml) and resistant (MIC≥16/4 μg /ml). Unfortunately, there are still no commercial kits for determining CAZ/AVI antibacterial activity, so we believe that Kirby-Bauer is an economical method for CAZ/AVI. In this study, in order to screen optimal content AVI disk containing ceftazidime (30 μg), inhibition zone diameters of four kinds of ceftazidime (30 μg) disk containing different AVI content (0 μg, 10 μg, 25 μg, 50 μg) against 122 Escherichia coli, 188 Klebsiella pneumonia and 76 Enterobacter cloacae were tested, the CLSI broth microdilution method interpretation was used as the standard to estimate activity.

Conclusions
In conclusion, our study indicates that 30 μg/10 μg CAZ/ AVI disk is a more feasible choice to evaluate CAZ/AVI activity against Enterobacteriaceae, and disk diffusion breakpoint CAZ/AVI (30 μg/10 μg) we recommended for Enterobacteriaceae (susceptible ≥20 mm and resistant ≤11 mm) has excellent sensitivity and specificity. Those data will provide a rapid and accurate detection of CAZ/AVI activity against the major Enterobacteriaceae and is conducive to provide more effective guidance for treatment of infectious diseases.

Bacterial strains
A total of 386 Enterobacteriaceae isolates (188 Klebsiella pneumoniae, 122 Escherichia coli, 76 Enterobacter cloacae) were collected according to random selection within group (ESBL negative isolates were random collected from ESBL negative group, ESBL positive isolates were random collected from ESBL positive group, CRE isolates were random collected from CRE group) from two different regions of China (Chengdu and Chongqing), all microorganisms were isolated from patient specimens (such as Urine, blood, sputum and secretion) during treatment, and then those isolates have to be preserved for scientific research. All strains were identified by GN card (bioMerieux, Durham, NC, USA) and ID32 GN (bioMerieux, Durham, NC, USA).

CRE experiment
Minimum inhibitory concentration (MIC) of ertapenem, imipenem and meropenem were obtained with broth microdilution, those isolates of which resistant to one or more carbapenems (ertapenem, imipenem and meropenem) using the current MIC breakpoints (CLSI, M100, 28th Edition, 2018) were defined as carbapenemresistant Enterobacteriaceae (CRE).

Ceftazidime/avibactam antimicrobial susceptibility test
Ceftazidime/avibactam MIC were tested by broth microdilution according to CLSI guideline (M100, 28th Edition, 2018) using a fixed concentration of avibactam (MedChemExpress. New Jersey. USA) 4 μg/ml. Each isolate was tested in duplicate; a third replicate was necessary if there was disagreement between the first two broth microdilution results. For Kirby-Bauer method, plates were incubated at 35°C and read after 16-20 h incubation, ceftazidime (30 μg) disk containing different avibactam content (0 μg, 10 μg, 25 μg, 50 μg) were used for disk diffusion test, k diffusion tests were performed in triplicate in parallel with broth microdilution, the mean value of the three inhibition zone diameter is used for statistical analysis. Escherichia coli ATCC 25922 and Escherichia coli ATCC 32518 were used for quality control. Isolates were considered to be susceptible to CAZ/ AVI when MICs were ≤ 8/4 mg/L (CLSI, M100, 28th Edition, 2018).

Statistical analysis
T-test was employed to assess the statistical significance of differences intra-group comparisons using GraphPad Prism 5 software (GraphPad Software, Inc., La Jolla, CA, USA). Differences were considered statistically significant at p < 0.01.
Additional file 1: Table S1. The zone diameter range with MIC.