K pneumoniae was known as the common cause of respiratory tract infections, urinary tract infections (UTIs) and bloodstream infections (BSIs) [20]. K. pneumoniae typically colonize human mucosal surfaces, including nasopharynx and GI tract. The colonization rate varies among different body sites, also between the community-acquired (CA) K. pneumoniae and the hospital-acquired (HA) K. pneumoniae. It is estimated that the rate of CA nasopharynx colonization was about 11%. The rate in adults is typically higher than that in children [20]. However, the rate of HA nasopharynx colonization is slightly higher, up to 19% [21]. Compared to the nasopharynx, the CA GI colonization rate is estimated to be around 3.9~5.9% [9]. Furthermore, the HA GI colonization rate varies from 23% to 30% [22, 23]. It was reported that the GI carriage of K. pneumoniae was related to the subsequent HA infection [24]. In 2017, a study was established to explore the association between GI colonization and infection. Additionally, for the adults, it was reported that the rate of K.pneumoniae infection was much higher for the GI colonization patients compared with the patients who were culture-negative (16% vs 3%) [9]. However, for the neonates, colonization of the neonatal gut takes place immediately after birth [25]. When some pathogens colonize the gut, it may later on lead to the subsequent infection. Compared to the neonates who were non-colonized, the likelihood of the colonized-neonates developing subsequent infections was remarkably higher (24.8% VS 1.9%). For the hospitalized neonates, the percentages of nasopharynx and GI K.pneumoniae colonization were respectively 29% and 36.8% [26]. Furthermore, a study showed that the GI K.pneumoniae could invade and penetrate the intestinal epithelium, which indicated that GI K.pneumoniae could cause extraintestinal infections. This transcellular translocation mechanism is exploited by K. pneumoniae strains from the gut flora to cause systematic infection [27]. Although there was a close relationship between colonization and infections, the homologous relationship between the GI colonized isolates and extraintestinal isolates had not been reported yet.
In our study, all the isolates (43/43) were MDR K.pneumoniae, including 27 strains with carbapenem resistance, with an observed carbapenem resistance of 62.8%. The proportion was moderately higher than what published by World Health Organization for the adults, which was 54% [28], while considerably higher than the proportions of 24.7% and 29.8% found in previous studies in the neonates [29, 30].
Here is one hypothesis indicating that, GI colonization is likely to be a significant reservoir in terms of transmission and infection [4], in addition, some drug resistant genes which were mediated by plasmids can be acquired or lost during bacterial translocation [20]. Based on this hypothesis, the drug resistant phenotypes might be affected by the loss or acquisition of the β-lactamase genes. In our study, the CTX-M-1, CTX-M-14, CTX-M-15 and TEM-1 were the different expression genes between feces and other samples. These genes belong to plasmid-mediated ESBLs [31]. In this case, the drug resistant genotypes and phenotypes were divided into different groups as per the antibiotic sensitivity tests and the expressions of the β-lactamase genes. The results showed that there were 12 patients (12/21, 57.1%) whose paired isolates might be homologous. The data demonstrated that the GI tract might be a significant reservoir for causing extraintestinal infections.
The majority of the isolates were resistant to β-lactam antibiotics. The resistance observed in the present study may be attributed to the expression of resistance genes such as β-lactamase genes. NDM-1 appeared in 7% of isolates, which was first identified in 2006. After it was first identified, it was predominantly found in K. pneumoniae and E. coli. Since 2010, the bacteria producing NDM-1 have been reported worldwide. In China, NDM-1 producing K.pneumoniae has been frequently reported in neonates [32,33]. The STs of blaNDM-1-producing K.pneumoniae mainly include ST11, ST16, ST20, ST37, ST70, ST147 and ST1419 [34~37]. But our data indicated that the ST54 was the only NDM-1 producing type. In most cases, bacteria with NDM-1 are resistant to almost all antibiotics. Moreover, the dissemination has been facilitated by horizontal gene transfer. That being so, reliable detection and surveillance are of the great importance in preventing the clonal outbreaks. Despite the fact that these isolates showed high drug resistance and high rates of resistance genes, just one neonate (patient 1) acquired a poor prognosis upon treatment with antibiotics.
To confirm whether the isolate pairs were homologous or not, a UMPGA tree was drawn by employing MEGAX to further analyze the homology among the different isolates from the same patients. Excluding the completely concordant strains, the analysis of the homology among ST37 and ST1083 should be confirmed. According to the analyses, ST37 and ST1083 were in the same cluster (two alleles of the 7 housekeeping genes differed), concluding that the two were closely related and the results validated a great deal of our previous research [19]. And our data indicated that ST37 were the main epidemic clones in the Neonate Ward, which showed consistency with what found in other studies [19, 38]. It was discovered that ST37 are presumably to be a potential high-risk MDR K.pneumoniae clonal lineage[39].Furthermore, it is reported that carriage of carbapenem-resistant K. pneumoniae (CRKP) in the GI tract may precede and possibly serve as a source for subsequent clinical infections in approximately 9% of carriers[40,41]. And these carriers may act as a significant reservoir for the dissemination of CRKP in the healthcare facilities [42~44].Combined with our study, active surveillance for detecting CRKP colonization is critical for preventing the CRKP from spreading. Besides, according to the guidance of CDC for control of Carbapenem- Resistant Enterobacteriaceae (CRE), screening rectal cultures of CRE is an important strategy for CRE prevention [45].