Epidemiology of Methicillin-Resistant Staphylococcus Aureus Bloodstream Infections from 2013-2017 at Chiang Mai University: What Changes Occurred from 2007-2011?

Background: Methicillin-resistant Staphylococcus aureus (MRSA) is an established pathogen that causes hospital- acquired infections worldwide. Bloodstream infection is associated with signicant morbidity and mortality. We conducted a study aimed at describing the epidemiology of MRSA bloodstream infections, to determine the minimal inhibitory concentration (MIC) of the antibiotic vancomycin among MRSA isolates, and to determine the rate and risk factors of mortality. Methods: A retrospective study was conducted among patients aged ≥ 18 years whose blood culture grew from to 2017 Results: The annual prevalence of MRSA in S.aureus bloodstream infections from 2013 to 2017 were 32.8, 23.1, 26.8, 19.2 and 15.4%, respectively. This prevalence showed a non-signicant decrease (p = 0.086). Eighty-four patients with 84 episodes of MRSA bloodstream infections were enrolled. Fifty-three patients (63.1%) were male, and the median age was 68.5 years (IQR 56, 79). Fifty-eight patients (69%) had bloodstream infections with other sites of infection: pneumonia (28 episodes, 43.1%), skin and soft tissue infections (16 episodes, 24.6%), osteomyelitis (7 episodes, 10.8%), infective endocarditis (4 episodes, 6.2%), septic arthritis (4 episodes, 6.2%), arterial graft infections (4 episodes, 6.2%), and urinary tract infections (2 episodes, 3.1%). Percentage of patients with vancomycin MICs ≥ 1.5 mg/L were 68.2%, 62.5%, 47.4%, 26.7%, and 75% from 2013 to 2017, respectively. (p = 0.325). The mortality rate was 64.3%. There was no signicant difference in mortality rate between those infected with MRSA with a MIC of vancomycin < 1.5 and ≥ 1.5 mg/L (p = 0.172). Factors associated with mortality included age ≥ 40 years old (OR 11.35; 95% CI: p 0.026), presence of of 11.19; 95% 2.83–44.18, p = 0.001) and concurrent (OR 4.44; 95% CI: 1.09–18.14, p = 0.038).

There was no signi cant difference in mortality rate between those infected with MRSA with a MIC of vancomycin < 1.5 and ≥ 1. Background Methicillin-resistant Staphylococcus aureus (MRSA) is an established pathogen that causes hospitalacquired infections worldwide. [1] MRSA bloodstream infection (BSI) is associated with signi cant morbidity and mortality. [2] The prevalence rate of methicillin resistance among S.aureus infections varies widely depending upon the study site and the year of the study. The SENTRY antimicrobial surveillance program collected 191, 460 S. aureus isolates from 427 centers globally. [3] From 1997 to 2016, the prevalence of MRSA among S. aureus ranged between 26.8% in Europe and 47.0% in North America. In the Asia-Paci c area, which includes Thailand, the prevalence was 39.6%. Among hospital-acquired S.aureus BSI, the prevalence of MRSA ranged from 37.8-45.3%. The ANSORP study group collected the isolates from 2004 to 2006 in Asian countries and found that the prevalence of MRSA among S.aureus isolates was highest in Sri Lanka (86.5%) and lowest in India (22.6%). [4] In Thailand, the prevalence of MRSA among S.aureus isolates from university hospitals from 2005-2006 was 57%, [4] and from 2012-2015 was 46.0%. [5] A study from northeastern Thailand reported the prevalence of MRSA among S. aureus BSI during 2006-2007 was 28%. [6] A prevalence of MRSA among S.aureus BSI of 23-43% during 2007-2011 from northern Thailand was also reported. [7] Vancomycin is the mainstay treatment for MRSA infection. However, infections caused by MRSA with reduced susceptibility to vancomycin have been reported including some cases in Thailand. [8][9][10][11][12] In addition, there have been increasing reports about patients who had treatment failure from vancomycin, despite their vancomycin MIC being ≤ 2 mg/L. [13][14][15] The increase in vancomycin MIC but within the susceptible range of MRSA is called the creep phenomenon. [16] Patients with MRSA BSIs with an elevated vancomycin MIC (≥ 1.5 mg/L) had poor clinical outcomes i.e. longer durations of bacteremia, higher recurrence rate of MRSA bacteremia after vancomycin discontinuation, longer hospital stays, and increased mortality. [17][18][19] In addition, the number of patients infected with MRSA with a vancomycin MIC ≤ 0.5 mg/L was associated with treatment success if compared with those infected with strains of vancomycin MIC of 1-2 mg/L. [20] We conducted this study along the same lines as the one conducted from 2007-2011 which aimed to describe the epidemiology, characteristics, mortality rate, and risk factors of death among patients who had MRSA BSIs. In addition, we also determines vancomycin MIC among MRSA isolates from hemoculture. Vancomycin MIC was routinely performed for blood cultures growing MRSA in our hospital from 2012.

Methods
A retrospective study was conducted at the Maharaj Nakorn Chiang Mai hospital, a 1400-bed, university a liated hospital in northern Thailand. Adult inpatients aged ≥ 18 years and had MRSA grown from blood culture from January 2013 to December 2017 were enrolled. Demographic and clinical characteristics, laboratory information, and microbiological data were collected.

Microbiological methods
Bacterial identi cation and antimicrobial susceptibility testing were performed at the Microbiology unit, Diagnostic laboratory, Maharaj Nakorn Chiang Mai hospital. S. aureus was identi ed by conventional biochemical tests. All S. aureus isolates were tested against 5 antibiotics including oxacillin (using cefoxitin as a surrogate drug), vancomycin, trimethoprim-sulfamethoxazole, erythromycin and clindamycin by disk diffusion method following the Clinical and Laboratory Standards Institute (CLSI) guidelines, M02-A11 [21] and M02-A12. [22] The disk diffusion results were interpreted according to the breakpoints stated in the CLSI guidelines, M-100 series. [22] A further minimal inhibitory concentration (MIC) test of vancomycin was performed among the MRSA samples isolated from blood cultures. MIC is the lowest concentration of a chemical which prevents visible growth of a bacterium. According to the CLSI, MRSA can be categorized into 3 groups: MIC ≤ 2 mg/L refers to vancomycin-susceptible S.aureus; MIC 4-8 mg/L refers to vancomycin-intermediate resistant S. aureus (VISA), and MIC ≥ 16 mg/L refers to vancomycin-resistant S.aureus (VRSA). [22] Statistical analysis Data were presented as numbers (%), means and standard deviation (SD), or medians and interquartile ranges (IQR) as appropriate. Characteristics between groups were compared using the Student's t-test, Mann-Whitney U test, Chi-square test or Fisher's exact test as appropriate. Factors associated with mortality were analyzed using a univariate logistic regression model. Variables with a p-value < 0.10 from the univariate analysis were then tested in a multivariate logistic regression model using a backward stepwise procedure. A two-sided test with a p-value of < 0.05 was used to determine statistical signi cance. All statistical analyses were performed using Stata statistical software version 10

Treatment and outcome
Seventy-nine patients received anti-MRSA treatment; 72 patients (85.7%) received vancomycin, 5 patients (6.0%) received linezolid, and 2 patients (2.4%) received fosfomycin. The median time from positive blood culture to receipt og appropriate antibiotics was 1 day (IQR 0, 3). The other 5 patients did not receive anti-MRSA treatment and died.
Fifty-four patients died giving a mortality rate of 64.3%. The patients who died were more likely to be older, had had prior use of piperacillin/tazobactam within 3 months, presence of alteration of consciousness, tachypnea, presence of medical devices, had concurrent pneumonia, and lower platelet count. Patients who survived were more likely to have an underlying condition of chronic kidney disease. (Tables 3 and 4)

Discussion
This study demonstrated that the prevalence of MRSA among S.aureus BSIs was non-signi cantly decreasing over this 5-year period, from 33-15%. This nding is in contrast with our study conducted between 2007 and 2011, in which there was an increasing prevalence, doubling from 23-43%. Demographic and clinical characteristics are comparable to the previous reports. The majority of patients had underlying diseases, had prior exposure to antibiotics within 3 months, and had medical devices inserted. Seventy percent of patients had concurrent other infections; 33% of patients had pneumonia and 19% of patients had skin and soft tissue infection. [6,7,24]. All clinical isolates were sensitive to vancomycin according to vancomycin MIC.
Vancomycin is a life-saving medication, and has been the mainstay of treatment for MRSA infections for several years. Although new medication i.e. linezolid, daptomycin, and ceftalorine is available, [25] they are not widely accessible for the majority of patients in Thailand, mainly due to high cost. Vancomycin has a narrow therapeutic index, high levels can cause toxicity and low levels can result in treatment failure. [26] Therefore, drug level must be monitored for patients receiving vancomycin. A level of 15-20 mg/L is required for treatment success of BSIs. [26,27] The pharmacokinetics (Pk)/ pharmacodynamics (Pd) parameter of vancomycin, which is the area under the curve (AUC) above the minimal inhibitory concentration (MIC) of > 400 is correlated with good clinical outcome. [27] However, in the era of vancomycin MIC creeps, higher drug levels may be required to achieve that Pk/Pd parameter. In the past, vancomycin MICs were mostly < 0.5 mg/L. [28,29] however, many reports demonstrated that vancomycin MICs are increasing. [16,[28][29][30]  Consent for publication: