Carbapenems were once the first choice of treatment for Ab infection. However, their long-time use might lead to the high expression of efflux pump-related genes in Ab and thus selectively induce the production of CRAB strains [15]. In China, the percentage of CRAB strains in Ab strains that have been clinically isolated and cultured has exceeded 50% for several successive years. More seriously, most CRAB strains are resistant to both penicillin and other β-lactamase antibiotics [23,24,25]. This, plus the limited medications available for children, means pediatric physicians, particularly those in the PICU, have to deal with a high incidence rate of hospital-acquired Ab infections, which can lead to difficulty in the selection of antibiotics when treating CRAB. This is the major cause for the high death rate, expensive treatment costs, and long-time hospitalization of patients with CRAB infections.
We first completed the statistics for all patients with an Ab-positive culture result in the past seven years. By screening, 105 patients were suspected of having had VAP caused by hospital-acquired Ab infection when aged < 36 months; their primary disease was mostly infectious disease, predominantly pulmonary and intracranial infections, and trauma. The analysis according to group demonstrated that the percentage of patients with diseases derived from pulmonary infection in the CRAB group (< 50%) was markedly lower than that in the CSAB group, while the percentage of patients with diseases derived from intracranial infection and trauma was significantly higher (about 1/3). Based on the statistics of various invasive procedures except mechanical ventilation, the percentage of patients undergoing bronchoscopic procedures in the CRAB group was lower than that in the CSAB group, while the percentage of patients undergoing other invasive procedures (including deep venous catheterization, thoracic puncture, abdominal puncture, urethral catheterization, and surgery) was significantly greater. Thus, it can be seen that the diversity of invasive procedures is a high risk for CRAB infection. In addition, a careful statistical analysis of the influence of primary disease severity, complications, body immunity, different treatment measures (especially invasive procedures) and antibiotics, mechanical ventilation time, and other possible factors on CRAB susceptibility in different patients was not undertaken because of the small sample size used. In addition, this was a retrospective study in which the collection of complete data was difficult, which might have caused a bias in results. In future, we shall pay particular concerns to patients with pulmonary infection, and shall step up our vigilance of patients on long-term mechanical ventilation who have an intracranial infection and trauma to prevent the occurrence of nosocomial infection.
A PELOD2 score is a recognized measure to evaluate disease severity and the prognosis of patients in comprehensive PICUs. The studied patients had a low PELOD2 score (median: 2) within 24 h after admission. The death rate (as calculated with the prediction formula of the death rate) was only 0.3% [26], but was > 25% as shown by our final statistical results. Such a large difference in the death rate may be related to a lack of successive scoring in our study in our study; it also laterally reflects that most patients in our statistics had no severe diseases at admission. However, the death rate greatly increased once these patients developed VAP caused by Ab infection. This is an explanation of why PICUs need to minimize invasive mechanical ventilation time and good airway care to avoid VAP. In the present study, although a statistically significant difference in the length of hospital stay and death rate between two groups was not observed, the mechanical ventilation time and post-diagnosis administration time for antibiotics were markedly longer in the CRAB compared to CSAB group; this indicates that VAP caused by CRAB infection is more difficult to treat than that caused by CSAB infection.
In this study, Ab strains in the CRAB group were resistant to nearly all β-lactamase antibiotics, with and without the addition of enzyme, and were only sensitive to amikacin, cephazolin, compound sulfamethoxazole and tigecycline; nearly 28% were XDR and PDR strains. More seriously, among several antibiotic options, aminoglycosides, sulfonamides, quinolones, and polymyxins are limited or should be carefully used in children. Aminoglycosides have such side effects as ototoxicity, nephrotoxicity, and neuromuscular blockade; their application, especially in infants, may induce unobservable ototoxicity symptoms, and further cause irreversible deaf-mutism if no drug discontinuation or intervention is implemented. Different types of aminoglycosides have a varying incidence rate of ototoxicity; in our study, the ototoxicity incidence rate of amikacin was 1.5% in the drug sensitivity test [27]. Tetracyclines can influence the teeth of infants and children; infants are more susceptible to such an impact, with even the short-term use of such drugs in infants under one years of age showing a high incidence of tooth invasion. In addition, tetracyclines can also induce skeletal deformation, osteosis suppression, infantile skeleton growth inhibition, and transient growth retardation. Therefore, tetracyclines are forbidden in children under eight years of age. It was found in animal experiments that quinolones can induce cartilage disease and articular toxicity in infantile rats and dogs; thus, these drugs are only recommended for patients ≥ 16 years of age as per manufacturer’s instructions. Polymyxins are the last line of defense for Ab infection because they are not only expensive, but also show tubular cytotoxicity that can lead to acute tubular necrosis [28].
Although the use of the above antibiotics is somewhat limited in pediatric departments and may cause various adverse reactions, pediatric clinicians often choose off-label or the combined use of antibiotics to treat CRAB infection after weighing the benefits and risks. This, however, may lead to an adverse effect on the subsequent growth and development of patients and may also result in a medical dispute. Ab strains in the CSAB group were sensitive to nearly all the 11 remaining antibiotics, excluding cefepime and ampicillin, and therefore treatment was less difficult. This might be one of the reasons for the significantly shorter administration time of antibiotics after Ab-positive culture in the CSAB group. Interestingly, all Ab strains in the CSAB group were sensitive to cephazolin as a first-generation cephalosporin, with strains in the CRAB group also having a sensitivity rate of nearly 57.4%. However, the sensitivity rate of strains to cefepime as a fourth-generation cephalosporin was only 37.8% in the CSAB group and nearly 0% in the CRAB group. Reviewing the application of antibiotics in our department and the purchase of antibiotics in our hospital during the past 10 + years, we suspect that the above phenomenon might be associated with an extremely low application rate of cephazolin and the extensive use of cefepime. This suggests that we perhaps will have more antibiotic options to treat CRAB infections with if selectively and protectively using several antibiotics in our daily work.
A. baumannii, as a non-fermenting bacterium, has a long survival time in the natural environment, a rapid and diverse capability of acquiring drug resistance, and varying genotypes. It is thus one of the bacteria most difficult to treat in the clinic. Therefore, it is important for the early diagnosis of Ab and its treatment with appropriate antibiotics to understand the drug resistance mechanisms of Ab and Ab genotypes in local regions and hospitals. Based on differences in gene sequence, β-lactamases are classified into four types: A, B, C, and D [29]. Type A β-lactamases are a kind of cephalosporin hydrolase that mainly produce clavulanic acid; type B β-lactamases are MBLs and their catalysis requires zinc or other heavy metals, while the catalysis of types A, C, and D lactamases depends on serine. MBLs with extensive substrates can induce the hydrolysis of almost all β-lactamase antibiotics, including carbapenems, but have no effect on monocyclic β-lactamase antibiotics. Type C β-lactamases are mainly resistant to cephalomycin, penicillin, and cephalosporins. Type D β-lactamases, also known as OXAs, can usually rapidly hydrolyze oxacillin. Currently, over 400 OXA enzymes have been identified, of which most have the capacity to hydrolyze carbapenems. In summary, types B and D β-lactamases serve as one of the major mechanisms for Ab resistance to carbapenems [25].
IPM and VIM detected in our study are the common genotypes of MBLs [29]. In the present study, the expression rate of IPM was > 80% in both CRAB and CSAB groups. However, a significant difference in the expression level of IPM between the two groups was not noted. In both groups, however, VIM was rarely expressed in Ab strains. Therefore, we presumed that the drug resistance mechanism of CRAB strains in the PICU of our hospital was not tightly related to MBLs. In the OXA series, OXA-23, OXA-24, OXA-51, and OXA-58 are the most common genotypes of Ab, but their expression varies markedly in different countries and regions. For example, in 2012, the high expression of OXA-58 in clinical Ab strains was reported in South Brazil [11]; in the Guangdong Province of China, OXA-58 was reported to be highly expressed in Ab strains isolated before 2008, while OXA-23 was mainly expressed in those isolated after 2009 [10]. In the past seven years, OXA-58 was rarely expressed in Ab strains from the PICU of our hospital. Similar to the study in Guangdong Province, the high expression of OXA-23 was also found in MDR-Ab strains from ICUs in Northwest China [12]. Meanwhile, a low expression level of OXA-23 in CSAB strains was also observed in another study [30]. OXA-23 had a high expression rate in Ab strains from the PICU of our hospital, with its expression level in the CRAB group significantly higher than that in the CSAB group. The expression trend of OXA-51 was nearly consistent with that of OXA-23, which is similar to recent study results at Chinese and abroad [13, 31]. In contrast, OXA-24 had a high expression rate in Ab strains from both groups, but its expression level in the CRAB group was lower than that in the CSAB group. This differs from the results of most studies showing that OXA-24 was highly expressed in MDR-Ab [14, 17, 18, 32]. The thought was that this might be associated with a small sample size, but it might be that the expression trend of OXA-24 in Ab strains from the PICU of our hospital was indeed different from that of other regions; this needs to be validated in a subsequent larger-sized study.
In addition to types B and D β-lactamases, the efflux pump is one of the major mechanisms for Ab resistance to carbapenems and tigecycline. To date, four efflux pump-related gene superfamilies have been discovered [25, 33, 34]. AdeA, AdeB, AdeC, AdeF, AdeG, and AdeH all belong to the resistance-node-cleavage superfamily of efflux pump-related genes. AdeABC is mostly associated with aminoglycoside resistance, and can reduce Ab sensitivity to tigecycline and non-fluoroquinolone antibiotics [35,36,37]. AdeFGH is related to tigecycline resistance [38]; meanwhile, a study [39] showed that AdeFGH overexpression induced by low-dose antibiotics was helpful for the biofilm production of Ab strains and increased the drug resistance of Ab. Therefore, we focally selected AdeA, AdeB, AdeC, AdeF, AdeG, and AdeH for genotype testing. Our final results indicated that the abovementioned genes showed high expression in all Ab strains from the PICU of our hospital, with expression levels significantly greater in the CRAB compared to CSAB group. These findings show that the high expression of AdeABC and AdeFGH may be involved in inducing the production of CRAB strains.