This study examined the incidence and characteristics of Gram-positive bloodstream infections in one of the largest tertiary-care hospitals in China. So far many studies have reported epidemiology, species distribution and antibiotic resistance of Staphylococci, Enterococcus spp. Streptococcus spp. Gram-positive bacilli and others [18,19,20], neverthless, only few studies have focused on Gram-positive bloodstream infections .
In our 6-year study, the incidence of Gram-positive BSIs switched order from 4.6 to 7.3 episodes per 10,00 admissions over 6 years. Acoording to our knowledge, thus far no such data, i.e. incidence rates of Gram-positive BSIs based on large retrospective studies have been reported. In the present study, a downward trend was observed in overall incidence in the ICU per year, while there was no obvious change on general ward. This might be explained with the increased importance of prevention and control infection among medical staff of ICU that was obvserved over recent years. Nevertheless, two recent studies haved reported an increase in the incidence of Gram-positive BSIs in general ward and ICUs [22, 23].
BSIs are commonly associated with comorbidities, such as malignancies, diabetes mellitus and infections [24, 6]. In the present study, we found that malignancy was the most common comorbidity, and the major predisposing factor for BSIs was indwelling central intravenous catheters; this data was consistent with previous results [25, 26]. Several studies have reported that the second most common factors is abdominal and lower respiratory tract infections [27, 18]; while we found that chemoradiotherapy was the second most common factor for BSIs. The observed differences might be due to the fact that the malignancy was the most common comorbidity with extensive use of intravascular catheters for chemotherapy and the high rate of radiotherapy, which more commonly lead to BSIs.
In our study, 69% of infections were healthcare-associated while 31% were community-associated (69% VS 31%). We hypothesized that this occurred due to the following reasons: first, the study population was mainly focused on healthcare-associated infection, and blood collection and culturing was a routine examination for hospitalized patients with fever. Another reason was that the common predisposing factors of Gram-positive BSIs such as central intravenous catheters, immunosuppression, and chemoradiotherapy were mainly found with hospitalized patients in our study. Beyond that, we have found that a rate of Streptococcus pneumoniae infection in China is significantly lower compared to other developed countries . Pneumococcal vaccine is routinely used to treat streptococcus pneumonia disease in developed countries, which might affect the morbidity and mortality . Nevertheless, China is still facing great challenges, such as uncomplete network for monitoring infection with streptococcus pneumoniae and lack of relevant immune policy for standardized use of pneumococcal vaccine. Moreover, besides PPV23 there is no available vaccine in China.
In our study, Staphylococci was 100% sensitive to vancomycin and linezolid except for Staphylococcus epidermidis, for which the vancomycin resistance rate was 0.13%. These results were consistent with the studies by Fayez et al. . However, Mamishi and colleagues have reported an obviously higher prevalence of resistance to penicillin by Streptococcus pneumonia, i.e. 42.86% in Asian countries . The reason might be the wide use of the variety of interventional procedures, immunosuppressive agents and vancomycin, so it is necessary to strengthen monitoring drug resistance and rational use of antibiotics in clinical. All of the cases of Staphylococci showed more than 90% resistance to penicillin except for MRSA. However, Vasudeva has reported that all Staphylococci showed are 100% resistant to penicillin [32, 33]. This may be related to the different practical uses of β-lactam antibiotics in various areas, resulting in different drug resistance to bacteria. Mover, the resistance rates of Staphylococci to gentamicin and tetracycline were less than 50%, Staphylococcus epidermis which was consistent with the previous studies [21, 34]. Therefore, both gentamicin and tetracycline might be good treatment choice against Staphylococci used to decrease the overwhelmingly dependent on vancomycin and linezolid.
In our study, all of the vancomycin-resistant isolates were E. faecium (4.1%), which was lower compared to number of other studies from US and Europe [35, 36]. Both E. faecalis and E. faecium have shown resistance to linezolid (3.8% and 3.1%), which was similar to the results from two studies from China [18, 21], nonetheless, resistance to linezolid was rarely reported in foreign studies . This may be explained with the species distribution and antimicrobial resistance that varies geographically. In addition, there were 219 (44%, 95%CI 38–50%) patients who were treated with effective antibiotics before obtaining the report of antibiotic susceptibility. Test reports have shown that patients with E. faecalis BSIs are more likely to receive effective treatment by the empirical use of antibiotics (61% vs 35%, p = 0.013). This may be explained by the low resistance rate of E. faecalis to many antimicrobial agents, such as penicillin (9.1%), ampicillin (11.4%), gentamicin (31.4%), and ciprofloxacin (35.6%).
In our study, there was no vancomycin and linezolid resistance to streptococcus pneumoniae and viridians streptococci, which was consistent with the studies done by Fayez and Marshall [30, 33]. Viridians streptococci was 100% sensitive to penicillin, but two other studies have reported that penicillin resistance rates have increased to more than 50% [38, 39]. The penicillin resistance rate of Streptococcus pneumoniae was 30%, similar to many European countries (20% to 50%), although much higher rates were reported in other Asian countries [40, 41], while there were only a few countries with lower rates (less than 10%) [9, 32, 42]. Therefore, the penicillin aslo resulted as a treatment choice for Streptococcus spp.
In our study, the crude mortality of Gram-positive BSIs and the mortality of Enterococcus spp. and Streptococcus spp. have shown a significant downward trend from 2011 to 2017. One possible reason might be that the number, correct timing and accuracy of blood cultures were all greater in a tertiary-care hospital thus medical workers could more correctly choose antibiotics in a timely manner. Further, there is a greater choice of antibiotics in non-tertiary hospitals. Another reason might be that tertiary-care hospitals have more complete strategy for strict control of infection that can reduce the mortality factors in BSIs. The mortality in the ICU has shown a decreased trend, while the general ward revealed contrary results. This could be explained by certain proportion of surgical and critically ill patients at our hospital, although many previous studies have shown that these patients have higher mortality rates than patients at the general ward [22, 23]. Nonetheless, the diagnosis and treatment technologies of ICU such as ventilator and ECMO have been rapidly developing rapidly over recent 5 years, and a variety of positive treatment measures have shown effective to improve cure rate and reduce mortality. Some other reasons that highlighted the importance of infection control measures among the medical staff of ICU are strict aseptic operations, good hand hygiene and similar. On the contrary, the use of indwelling central intravenous catheter seem to be increasing because of more chemotherapy and venous nutrition employed at general ward, while the consciousness of medical staff related to BSIs remained weak and the maintenance measures related to central intravenous catheter remained not enough strict. Aslo and the patients at general ward later received the empirical treatment thus further increasing the mortality.
There are several limitations in the present study that should be considered. First, the collection of clinical data depended on medical records rather than interviews and the comprehensive assessment of clinical symptoms. Second, there was an inevitable bias this was a single-centre study, and some of the results might have been affected by the small sample size. Finally, many drug susceptibility results could not be evaluated because of a lack of unified standards aslo and not all isolates underwent the same antimicrobial agent sensitivity tests, so many isolates lacked resistance comparisons with other isolates in the present study.