Risk Factors Of Nosocomial MRSA Infection And Molecular Characteristics Of Staphylococcus Aureus Isolated From Blood

Section 1 Risk factors of nosocomial methicillin-resistant staphylococcus aureus infectionObjective To investigate the risk factors of methicillin-resistant Staphylococcus aureus infection, so as to provide the evidence for the measures of MRSA control procedures.Methods A total of 118 MRSA strains and 116 MSSA strains were isolated from hospitalized patients in a general hospitial from January 1, 2013 to December 31, 2014. All surveillianced MRSA patients were enrolled in case group, and MSSA patients were enrolled in control group. Data of relevant exposure factors was extracted by self-made questionnaire. Relevant exposure factors included age, underlying diseases(cardiovascular and cerebrovascular diseases,diabetes,cancer,structural lung disease, hepatic and renal insufficiency), length of stay, invasive procedures and consumption of antibiotics. Univariate and multivariate analysises were using X2 test and nonconditonal logitistic regression model for non-matched case-control study.Result 1) 118 MRSA infectors, 116 MSSA infectors were enrolled in the study. 2) 32.21% MRSA isolations were from department of neurology and neurosurgery,25.86% MSSA isolations were from department of orthopedics. Most of MRSA was isolated from sputum, while most of MSSA was isolated from wound secretion. 3) The length of stay of MRSA group was longer than that of MSSA group. 4)Univariat analysis of risk factors for MRSA bacteraemia using MSSA bacteraemia controls show that older, central nervous system disease, the number of underlying diseases, invasive procedures(arterial/venous catheterization, tracheostomy/intubation, indwelling nasogastric tube, indwelling urinary catheter, postoperative drainage),the number of invasive procedures, the length of stay before Staphylococcus aureus isolation, the time and the number of antibiotic exposure maybe the risk factor of MRSA infections when compared with MSSA patients( P<0.05).Multivariate analysises showed that the main risk factors related to nosocomial MRSA infection included the number of invasive procedures, exposure to more than 2 kinds of antibiotics, antibiotics used more than 7 days(P<0.05).Conclusion The number of invasive procedures, exposure to more than 2 kinds of antibiotics, antibiotics used more than 7 days are independent risk factors of nosocomial MRSA infection when compared with MSSA infection. Consequently, avoiding unnecessary invasive procedures and reasonable application of antibiotics are the important means for prevention of nosocomial MRSA infection.Section 2 Molecular characteristics of methicillin-resistant staphylococcus aureus and methicillin sensitive staphylococcus aureus isolated from bloodobjective To study the clinical and molecular characteristics of methicillin-resistant Staphylococcus aureus and methicillin sensitive staphylococcus aureus isolated from patients’ blood,enhance the understanding of the drug resistance and transmission mechanism of MRSA.Method A total of 19 strains of MRSA and 13 strains of MSSA were isolated from hospitalized patients’ blood in our hospital from Jan. 2011 to Dec. 2011. The clinical characteristics and drug resistance were investigated. Both the MRSA and MSSA strains were tested for susceptibility using agar dilution method. Furthermore, 55 virulence factors(adhesin sas X, fnb A, clf A, clf B, ica A, cna, efb, isd A, isd B, isd C; cytotoxin hla, hlb, hld, hlg, hlg-2, pvl, luk E, luk M, psm-mec; enzyme ssp、sak、nuc、hys A、lip; superantigen sea, seb, sec, sed, see, seg, seh, sei, sej, sek, sel, sem, sen, seo, sep, seq, eta, tst; exotoxin set1, set2, set3, set4, set5, set6, set7, set8, set9, set10, set11; capsular antigen cap5, cap8) and 7 drug resistance genes(mec A,aac(6’) /aph(2”),aph(3’)-Ⅲ,erm A,erm B,erm C,tet M)were determined by PCR, and positive PCR products were sequenced. Index cluster amalysis between the staphylococcus aureus strains was conducted by UPGMA.Result 1) The mean age of the group of MRSA was older than the group of MSSA(78.5±7.6 years vs 67.6±15.3 years,P<0.05). The duration of MRSA identification was longer than MSSA, it was 9.84±4.11 days vs. 6.93±3.26 days(P<0.05)from the date of patients admission. 2) 32 Staphylococcus aureus strains were all susceptive to vancomycin. MRSA resistance rates of 12 kinds of antimicrobial agents were OXA(100.0%), PEN(100.0%), AMP(100.0%), CZO(100.0%)>CIP(94.7%)>ERY(84.2%)>QDA(73.7%)>TCY(52.6%)>GEN(52.6%)>RFP(26.3%)>SMZ/TMP(21.1%)>VAN(0.0%). 3) Six types of virulence genes(adhesin, cytotoxin, enzyme, superantigen, exotoxin, capsular antigen)were all detected in 32 Staphylococcus aureus strains. The positive rate of cna, luk E, psm-mec, ssp, nuc, lip, seg, sem, sen, set6, set11 was different between MRSA and MSSA, respectively(P<0.05). 4) there was difference in resistant genes between MRSA and MSSA. The positive rate of mec A, aac(6’) /aph(2”), aph(3’)-Ⅲ, tet M was different between MRSA and MSSA, respectively(P<0.05). erm A and erm B were not detected in MRSA group and MSSA group. 5) Index cluster analysis showed that bacteria cloning existed in 32 strains. The phenomenon indicated nosocomial infection.Conclusion The high positive rates of adhesin genes manifest the characteristic of colonization. MRSA strains carry more virulence genes than MSSA, which indicates the ability of injuring host tissue. Nosocomial infection control management should be performed in order to reduce the risk of infection.