Recovery of Multidrug-Resistant Bacteria from Swabs Stored for Durations of 1 and 4 Weeks under Conditions Mimicking Long-Distance-Shipping Conditions

LETTER

The study of environmental contamination or patient colonization is difficult in resource-poor settings, such as deployed combat or humanitarian facilities. Previous studies have relied on the use of swabs sent at great distances to laboratories for culture, with the associated uncertainty of the accuracy of the recovery results (1, 2). Even in settings where local laboratories are present, collected specimens may be referred out for processing and culture. To simulate the delays and shipping conditions that are potentially encountered using this method, we studied the recovery of bacteria from swabs that had been inoculated with multidrug-resistant (MDR) bacteria and stored at three different temperatures for 1 and 4 weeks.

CultureSwab Plus and CultureSwab MaxV(+) Amies gel without charcoal culture swabs (BD Diagnostics, Sparks, MD) [here called Plus swabs and MaxV(+) swabs] were included in this study. Both types of swabs are composed of rayon fiber; the MaxV(+) swabs are embedded with a proprietary mix of proteins to maximize recovery. MDR bacteria included Acinetobacter baumannii, Klebsiella pneumoniae, Escherichia coli, Pseudomonas aeruginosa, and methicillin-resistant Staphylococcus aureus (MRSA). Swabs were immersed in saline solutions containing 10² or 10⁵ organisms/ml and then stored for 1 day, 1 week, or 4 weeks at 4°C, room temperature (RT), or 45°C. After storage, swabs were streaked onto blood, MacConkey, and chromogenic (S. aureus) media, incubated overnight at 35°C, and observed for the presence and quantity of growth.

All five MDR bacteria were recovered at 1 and 4 weeks (Table 1). All bacteria were recovered at both concentrations stored at RT for 1 and 4 weeks, with the exception of low-concentration MRSA, which was not recovered from Plus swabs. At 45°C, A. baumannii and P. aeruginosa were not recovered from either swab at 1 or 4 weeks; MRSA was not recovered at this temperature from Plus swabs. After storage at 4°C, with the exception of K. pneumoniae from Plus swabs at 1 week, all MDR bacteria were recovered only at the higher concentration at 1 and 4 weeks.

TABLE 1.

Recovery of MDR bacteria from swabs stored at various temperatures for 1 and 4 weeks to simulate delayed culture due to transportation

Organism	Swab	Inoculum (CFU/ml)	Growth of bacteria after recovery at:a
			1 wk			4 wks
			4°C	RT	45°C	4°C	RT	45°C
A. baumannii	Plus	102	−	+++	−	−	+++	−
	Plus	105	++	+++	−	++	+++	−
	MaxV(+)	102	−	+++	−	−	+++	−
	MaxV(+)	105	++	+++	−	++	+++	−
K. pneumoniae	Plus	102	+	+++	++	−	+++	++
	Plus	105	+	+++	++	+	+++	++
	MaxV(+)	102	−	+++	++	−	+++	++
	MaxV(+)	105	+	+++	++	+	+++	++
E. coli	Plus	102	−	+++	++	−	+++	++
	Plus	105	+	+++	++	+	+++	++
	MaxV(+)	102	−	+++	++	−	+++	++
	MaxV(+)	105	+	+++	+++	−	+++	++
P. aeruginosa	Plus	102	−	+++	−	−	+++	−
	Plus	105	++	+++	−	++	+++	−
	MaxV(+)	102	−	+++	−	−	+++	−
	MaxV(+)	105	++	+++	−	++	+++	−
MRSA	Plus	102	−	−	−	−	−	−
	Plus	105	++	+++	−	+++	++	−
	MaxV(+)	102	−	++	+++	−	++	+++
	MaxV(+)	105	+	++	+++	++	+++	++

^aRT, room temperature storage; −, no growth; +, minimal growth; ++, moderate growth; +++, heavy growth.

Three previous studies have examined the ability of swab transport systems to recover bacteria after a time delay, but these studied delays of shorter durations and storage at lower temperature ranges. All three included the MaxV(+) swab system or its forerunner, the Copan M40 Transystem (3, 4, 5). One examined three swabs at room temperature held for up to 48 h (4). In another study, three swab types were stored at refrigerator (4°C) and room (22 to 24°C) temperatures for up to 48 h (5). That study documented the recovery of most tested bacteria at 48 h. The third study employed four swab types at room temperature for up to 7 days (3). In that study, some bacteria were recoverable out to 7 days with each swab.

Although we did not test all storage conditions that might occur during overseas or delayed shipment of specimens from remote sites of collection, our study clearly supports that MDR organisms can be shipped successfully, even if shipment is delayed up to 4 weeks, when the temperature is maintained between 4 and 24°C.