Difference on Cefoxitin and Oxacillin Disk Test on In Vitro MRSA Detection ( Meticillin Resistant Staphylococcus aureus ) Experimental Study on Microbiology Laboratorium of Medicine Faculty of UNISSULLA

Background: MRSA phenotypic detection has been a problem since it was found in 1962. Some studies explain that the diffusion method of cefoxitin and oxacilin disk test can be used to detect MRSA. Objective: knowing if there is difference between cefoxitin disk test and oxacilin disk test to detect


INTRODUCTION
Resistant Staphylococcus aureus towards meticillin antibiotics is known as Meticylin Resistant Staphylococcus aureus (MRSA) (Juuti, 2004).Those Stapyhlococcus aureus are resistant towards antibiotics meticylin because its ability to produce β-laktamase enzymeThis enzyme is able to eliminate antibacterial power especially in penicillin groups such as meticillin, oxacilin, penicilin G and ampicillin.(Juuti, 2004).MRSA phenotypic detection has been a problem since found in 1962 (Madhusudhan NS, et al., 2011).MRSA diagnosis is very important.Accuracy and reliability to detect meticilin resistance is the most important key to confirm antibiotic treatment for infected patient and to control MRSA staphylococci around hospital environment (Velasco, et al., 2005).MRSA resistance detection can be conducted by using oxacilin or cefoxitin diffusion method (Van Leeuwen WB, 2003;Broekema NM, et al., 2009).
A good diagnostic instrument can be recognized from its high sensitivity, specifity, Positive Predictive Value (PPV), and Negative Predictive Value (NPV).Study by Madhusudhan NS, et.al in 2011 using 100 MRSA speciments by diffusion method resulted that on detection using cefoxitin disk, 84 resistent.False positive value was11% and expected positive value of cefoxitin was 86.90% Jana M. Swenson, et.al studied on MRSA detection by dilution resulted that cefoxitin had the sensitivity and specivity value of 99,7% dan 100%.Oxacilin, which is on the same antibiotic group with meticillin, is cheaper and easily accessible (Van Leeuwen WB, 2003;David Velasco et al., 2004).The sensitivity of the oxacilin can be applied on other penicillinase-stable penicillin Oxacilin zone are often hazy and commonly misinterpreted as the result of oxacilin sensitivity (Pottumarthy, S., T. R. Fritsche, dan R. N. Jones, 2005).Cefoxitin can be used as MRSA detection both by diffusion or gel dilution (Clarence J. Fernandes, et al., 2005).Cefoxitin result is easier to be interpreted and more readable (Felten, A., 2002;Mimica, 2007Pottumarthy, S., T. R. Fritsche, dan R. N. Jones, 2005).Cefoxitin sensitivity on MRSA detection is mediated by mec-A gene (Swenson, J. M., et al., 2007).
Based on oxacilin and cefoxitin disks difference on MRSA detection, a research was conducted.This research aims to differentiate sensitivity, specivity , PPV, and NPV of cefoxitin disk test and oxacilin disk test to detect MRSA by diffusion method.

METHOD
This research is laboratorium experiment with specific method diagnostic test.Population of the study are Methicillin Resistent staphylococcus aureus (MRSA) and Methicillin Sensitif staphylococcus aureus (MSSA) bacteria collected from Microbiology Laboratorium of Rumah Sakit Umum Dr. Karyadi Semarang with density level of 0,5 Mc Farland (1,5 x 10 8 / ml) and 0,2 cc of volume embedded into 24 petri dish with muller hinton media.
Speciments used were 12 petri dishes with MRSA bacteria and 12 petri dishes with MSSA bacteria.Each dishes were tested with diffusion method on oxacilin disk and cefoxitin disk and resulted into 48 dishes.The amount of the speciments were counted from total sample formula.
Data analysis by fisher test were conducted to test research hypothesis with significance level of < 0.05.

RESULT
The result of cefoxitin disk to determine MRSA and MSSA were illustrated in the table below: Table 1.showed cefoxitin test resulted into 12 MRSA resistant speciments and no MRSA sensitive speciments.While for MSSA there are no resistant speciments.12 sensitive speciments were tested with fischer hypothesis test and acquired, p=0,000 (<0,05), meaning cefoxitin disk test is significant in MRSA detection.
Table 2. showed oxacilin disk resulted 9 resistant MRSA speciments and 3 sensitive MRSA speciments.While for MSSA there were no resistant speciments and 12 sensitive speciments were using Fisher test to test hypothesis resulted p=0,000(<0,05).This concluded that oxacilin disk test have significant value on MRSA detection.
This finding similar to previous study conducted by Clarence J. Fernandes, et al., 2005, which stated that sensitivity and specificity of cefoxitin are higher compared to oxacilin.So thatcefoxitin can be used for MRSA detection whether with diffusion or dilution method.(Clarence J. Fernandes, et al., 2005).The superiority of cefoxitin on MRSA detection is because cefoxitin act as strong inducer onmecA Gene regulatory system (Swenson JM, et al, 2007).Cefoxitin is easier to interpret and to read (Felten, A., 2002;Mimica, 2007Pottumarthy, S., T. R. Fritsche, dan R. N. Jones, 2005).MRSA resistance mechanism toward cefoxitin is because its difficulties to be broken by drugs; lossspecific penicillin binding protein (PBP); anddrugs degradation by betalaktamase (Yati & Gan, 2007).
Oxacilin, whichis also on the same antibiotic group with meticillin, is cheaper and accessible (Van Leeuwen WB, 2003;David Velasco et al., 2004).Oxacillinreplace metycilin which is no longer available commercially in the US and oxacilin is more possible to detect heteroresistant strain.Vulnerability result of oxacilin can be applied to penicillin group which are stable towards penisilinase, such as cloxasilin, dicloxacillin, methicillin, flukloxasilin dan naficillin.Oxaclin zone are often hazy and commonly misinterpreted as oxacilin sensitivity (Pottumarthy, S., T. R. Fritsche, dan R. N. Jones, 2005).MRSA resistance mechanism to oxacilin antibiotic was caused by betalaktamase enzyme formation; drug tolerancy due to failure in bacteria autolycine enzyme; bacteria which do not have celluler walls (mikoplasma), PBP changes or drugs unableto reach PBP (Yati & Gan, 2007).
MSSA detection by using cefoxitin disk as well as oxacilin disc showed that all 24 speciments were sensitive, confirmed by Short-Incubation Automated Instrument Systems (SIAIS.Detection of MRSA by cefoxitin disk showed that all 12 resistent speciments confirmed by SIAIS.But on MRSA detection with oxacilin disc showed that 9 speciments were resistant, while 3 speciments were sensitive confirmed by (SIAIS).These three different results possibly because oxacilin For example there was sputum and blood speciments.The differences of the speciments were not effecting the research validity.

CONCLUSION
Based on the research data on difference between cefoxitin disc and oxacilin disc on in vitro MRSA detection using diffusion method, it can concluded that there are significant difference between cefoxitin disc and oxacilin disc.Cefoxitin sensitivity to detect MRSA (96,2%) were higher than oxacilin (73,1%).Cefoxitin specificity to detect MRSA is similar to oxacilin (96,2%).Cefoxitin PPV to detect MRSA (96,2%) is higher than oxacilin disc (95,0%).Cefoxitin NPV to detect MRSA (96,2%) is higher compared to oxacilin disc(78,1%).Diffusion method on cefoxitin disk is better than oxacillin MRSA detection.Suggestions for further research are higher number of sample, same speciments materials, and comparison based on age and duration of the infection.

Table .
Cross tabulation of cefoxitin disk test

Table .
Cross tabulation of oxacilin disc zone are often hazy so it was misinterpreted as the evidence of oxacilin sensitivity (Pottumarthy, S., T. R. Fritsche, dan R. N.Jones, 2005)./jurnal.unissula.ac.id/index.php/sainsmedikaLimitation of this study was researcher only use disk diffusion test.It would be better if the antibiotic sensitivity test by dilution as antibiotic sensitivity test gold standard is used.Other constrains were speciments material collection from the patients are not similar.