Antimicrobial Ointments and MRSA USA300

In This Article

Abstract and Introduction
The Study
Conclusions
References

Abstract and Introduction

Abstract

We tested 259 methicillin-resistant Staphylococcus aureus isolates and 2 USA300 ATCC type strains for susceptibility to bacitracin and neomycin contained in over-the-counter antibacterial ointments. Resistance to both bacitracin and neomycin was found only in USA300. The use of over-the counter antimicrobial drugs may select for the USA300 clone.

Introduction

Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) is rapidly spreading worldwide. MRSA USA300 is a clone of increasing public health concern among rapidly disseminating CA-MRSA strains in the United States.^[1] MRSA USA300 is designated as sequence type (ST) 8 by multilocus sequence typing (MLST) and possesses staphylococcal cassette chromosome mec (SCCmec) type IVa. Although the rapid dissemination of the USA300 clone may occur because of a high virulence level that arises from the production of Panton-Valentine leukocidin (PVL) or an existing arginine catabolic mobile element (ACME) (2), there is no conclusive evidence to support this hypothesis.^[3] Furthermore, the hypothesis cannot account for the rapid dissemination of MRSA in countries where USA300 clones are not the dominant clones (most European countries, South Korea, and Japan).^[4–6] In most European countries, the dominant CA-MRSA clone is the European clone (ST80, SCCmec type IV, PVL positive and ACME negative).^[1,4] In South Korea, only 1 isolate was a USA300 clone among 138 MRSA isolates collected from patients with bacteremia and soft tissue infection.^[5] In Japan, the MRSA USA300 clone is rare.^[6]

In many cases, soft tissue infection acquired in communities was treated by using over–the-counter (OTC) drugs called triple-antibiotic ointment (TAO), e.g., Neosporin (polymyxin B [PL-B] sulfate, 5,000 units/g; bacitracin, 400 units/g; and neomycin, 3.5 mg/g) and Polysporin triple ointment (PL-B sulfate, 10,000 units/g; bacitracin, 500 units/g; and gramicidin 0.25 mg/g). These ointments contain antimicrobial drugs at concentrations far exceeding their MICs among S. aureus strains (16–32 μg/mL [equivalent to 124–248 unit/mL] for PL-B, <1–64 units/mL for bacitracin, and <1–>128 μg/mL for neomycin).^[7,8] It is hypothesized that CA-MRSA cases in the United States were under the selective pressure of TAOs.

In this study, we tested the susceptibilities of MRSA isolates, including the USA300 clone, to the antimicrobial drugs in TAOs. We also considered the possible role of TAOs in spreading the USA300 clone.

Cite this: Antimicrobial Ointments and Methicillin-Resistant Staphylococcus Aureus USA300 - Medscape - Oct 01, 2011.

Table 1. Source of methicillin-resistant Staphylococcus aureus isolates, Japan, 2004–2010
Source	Outpatients		Inpatients		Health care workers
Source	No. community-acquired infections*	No. hospital-acquired infections	No. community-acquired infections*	No. hospital-acquired infections	Health care workers
Skin and soft tissue	23	7	4	23	0
Bloodstream	0	2	1	19	0
Respiratory tract	0	2	1	17	0
Urinary tract	0	2	2	3	0
Ear	8	0	0	1	0
Eye	3	0	0	3	0
Others	1	1	2	3	0
Carriage	14	10	3	29	0
Screening	50	0	2	20	3
Total	99	24	15	118	3

*Community-acquired infections were determined on the basis of patients' histories according to Centers for Disease Control and Prevention (Atlanta, GA, USA) guidelines (www.cdc.gov/mrsa/diagnosis/index.html).

Table 2. Bacitracin and neomycin susceptibility of MRSA USA300 and other MRSA isolates*
Bacitracin/neomycin	MRSA USA300 (MICs of bacitracin, neomycin, and polymyxin B)†			Other MRSA
Bacitracin/neomycin	Kyoto University Hospital	Nagoya Medical Center	ATCC type strains	Other MRSA
R/R	9 (400, 128, 200–400)	0	BAA1717 (400, 128, 400)	0
R/S	0	1 (400, 0.25, 400)		0
S/R	0	0		11
I/S	0	0		1
S/I	0	0		132
S/S	9 (6.25–12.5, 0.25, 400)	0	BAA1556 (6.25, 0.25, 400)	96
Total	18	1	2	240

*MRSA, methicillin-resistant Staphylococcus aureus; ATCC, American Type Culture Collection (Manassas, VA, USA); R, resistant; S, susceptible; I, intermediate resistance.
†MICs are expressed as units/mL for bacitracin and polymyxin B and in μg/mL for neomycin.