PCNSIs were reported to be the most serious complications after craniotomy, with an incidence of varying from 0.9-7.4%, characterized by high incidence, morbidity and mortality [2, 8-11]. In present study, we reported an incidence of PCNSI was 1.5%, which was similar to those previous studies. In our study, only culture-positive patients were included, implying a lower false-positive rate and it might be the possible reason of relative low incidence. The initial symptoms of PCNSIs are difficult to distinguish by different responsible pathogens, such as bacteria, viruses, fungi, and parasites. A retrospective study revealed the bacterial meningitis accounted for more than 86% (BM) of CNSIs in China . This result was in line with our study, which showed the bacterial infection was the most prevalent subtype.
In table 1, ventricular drainage was the most common operation type in 23 PCNSI patients (9/23, 39.1%), which indicating that operation type might be a risk factor for the infection. It is important to adopt appropriate operation type, by decreasing operation time, carefully preventing drainage from pollution site and proper treatment could be effective.
Microbial culture and identification remain the gold standard for diagnosing bacterial meningitis [13, 14]. However, with the widespread applications of prophylactic antibiotics, the culture-positive rate of infection was relatively low, reporting from 7.5% to 50% [15-17]. In our study, all patients received 1.0 g cephalosporin as prophylactic therapy, which might lead to the relative lower CSF culture positive rate (29.1%). In addition, we set the diagnosis standard that the same species of microorganisms cultured from different isolates within 3 days were counted as a single isolate. Hence force, this may be another reason responsible for the low CSF culture positive rate. Moreover, different CNSIs diagnostic standards vary from study to study, which also could contribute to the low culture-positive rate.
Our study demonstrated Gram-positive bacteria was the most common organism, accounting for approximately 56% of the total isolates. This result was consistent with previous studies [12,18,19]. On the contrary, Aishwarya et al from India reported the proportion of Gram-negative bacteria causing PCNSIs was even as high as 91.6% . The proportion disparities lie in preoperative therapies, operative aseptic procedures and postoperative managements. However, there does exist a tendency that Gram-negative bacteria increase in prevalence, especially the Acinetobacter baumannii, which was the most frequent causative Gram-negative agent in our study.
Our results showed the coagulase-negative staphylococci were the most prevalent pathogens of Gram-positive bacteria (38%), which included Staphylococcus epidermidis (12%), Staphylococcus haemolyticus (6%), Staphylococcus capitis (9%), Staphylococcus warneri (6%) and Staphylococcus hominis (3%), followed by Enterococcus faecalis (6%). Twelve (92.3%) coagulase-negative staphylococci were methicillin-resistant coagulase-negative staphylococci (MRCoNS) and they were susceptible to vancomycin, rifampicin and linezolid. The results were mostly consistent with the results of previous studies [21,22]. In order to compare the efficacy and adverse effects of vancomycin versus cefazolin as antimicrobial prophylaxis for CSF shunt infection, E. Tacconelli et al found mortality of patients with post-surgical infections was higher in cefazolin group, implying the use of vancomycin could reduce the infections in the context of high prevalence of methicillin-resistant bacteria . Furthermore, vancomycin was also the last resort and the drug of choice to treat infections caused by Gram-positive bacteria in CNS infection.
Acinetobacter baumannii has emerged as an important nosocomial pathogen causing infections . The SENTRY Antimicrobial Surveillance Program reported that the frequency of Acinetobacter spp. was approximately 7% . However, there were 10 (29%) Acinetobacter baumannii isolates in our study, and all isolates were multidrug-resistant (MDR) bacteria with resistance to 14 out of 17 antimicrobials tested. The global antimicrobial surveillance study and the Tigecycline Evaluation and Surveillance Trial (T.E.S.T) showed that meropenem resistant Acinetobacter spp. increased from 17.7% to 33.0% and multi-drug resistant Acinetobacter spp. increased from 25.6% to 49.7% in 2005-2007 to 2008-2012 , and the rate of carbapenem-resistant Acinetobacter baumannii (CRAB) has risen in recent years . The emergence of MDR Acinetobacter baumannii, known as one of the ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, A. baumannii, Pseudomonas aeruginosa, and Enterobacter species) pathogens , has become a serious medical problem worldwide [29,30]. Acinetobacter baumannii has many resistance mechanisms, including ß-lactamases, aminoglycoside-modifying enzymes, efflux pumps, permeability defects, and modifications of target sites. The accumulation of several resistance mechanisms in A. baumannii has gradually decreased the number of antibiotic classes available to treat Acinetobacter baumannii infections in clinical practice. To combat the MDR Acinetobacter baumannii, combination therapies including colistin/imipenem, colistin/meropenem, colistin/rifampicin, colistin/tigecycline, colistin/sulbactam, colistin/teicoplanin, and imipenem/sulbactam should be treated. In addition, new non- antibiotic methods such as bacteriophage, has renewed interest in the therapy .
Klebsiella pneumonia is an opportunistic pathogen capable of causing a variety of infections, and we found two isolates (6%) in our study. The infection incidence differs from place to place and with different age groups [32,33]. The percentage were: 96 (68%) of 142 in Taiwan, 25 (43%) of 68 in the United States, 28 (39%) of 71 in Australia, 40 (34%) of 116 in South Africa, 6 (22%) of 27 in Europe, and 7 (17%) of 41 in Argentina . Alarmingly, multidrug-resistant of Klebsiella pneumonia was detected among the Gram-negative bacteria. Previous studies reported Klebsiella pneumonia have acquired antimicrobial resistance genes, and these strains encoded two plasmids with drug-resistance potential [34,35]. Widespread, highly strains armed with multidrug-resistant determinants are a great cause for concern. Changes of local microenvironments and combinations of antibiotics may improve the treatment efficacy.
The mortality rate of PCNSIs dropped drastically in past years, from 34% in 2005  to 19% in 2011 , and a recent study reported that the mortality was 3.6% . The mortality of present study was relatively higher than those reported, which was 21.7%. It could be explained by that only 23 patients were included by our restrict inclusion criteria and more data are required. Another possible explanation might be that the diagnosis of PCNSIs in this study depended on the isolation of a culture results rather than clinical signs, so that many infected patients without positive cultures might not be included. Therefore, the incidence, positive-culture rate, mortality and pathogens of PCNSIs vary between studies and are difficult to compare between different studies.
The significance of this study not only showed the distribution of pathogens from a region, but also reflected a similar trend to that observed in China. Clinicians would choose the appropriate empirical antibiotic therapy for PCNSIs according to the characteristics of the pathogenetic organisms. There are also several limitations. Firstly, the number of samples was not large enough to represent the whole situation of China. Secondly, we had no data for the analyses of Eumycetes, tuberculous and parasitic infections. Furthermore, the mechanisms of drug-resistance were not explored because of molecular detection was not performed. Notwithstanding its limitations, this study does provide an outline of bacterial prevalence of PCNSIs and the study continues to update data.