Laboratory Evaluation of the ESwab (ES) Transport System for Recovery of Multi-Drug Resistant (MDR) Acinetobacter baumannii (AB) from Fecal Specimens

Background: Detection of carriage of MDR-AB is highly important in management of nosocomial outbreaks. However, MDR-AB carriage at various anatomical sites is variable and may be difficult to detect. Rectal swabs may be a preferred method for detection of MDR-AB carriage. The ES is a new swab transport system utilizing a flocked nylon swab and liquid Amies medium, possibly superior over standard swabs (SS). However, ES has not been specifically evaluated for detection of AB carriage.

Objective: To evaluate the performance of ES for detection of AB in fecal specimens.

Methods: Normal fecal microbiota recovered from volunteer feces and 2 clinical carbapenem-resistant AB isolates were each suspended in sterile saline to the turbidity of 0.5MF. The 2 AB suspensions were further diluted 1:100 and 1:10^-4 and then mixed 1:1 with the fecal microbiota suspension (a total of 6 different inocula). ES and SS (Copan Diagnostics Inc., CA, USA) were suspended in 100µl of inoculum. The effects of 1h, 24h and 48h transport times (TT) at room temperature and storage conditions (1 week and 1 month of refrigeration (4°c) or deep-freeze (-80°c)) on the bacterial viability index (VI) were evaluated. Swabs were vortexed and eluted in saline. Elutes were plated onto MacConkey agar (MAC) containing imipenem 1µg/ml and control MAC plates. VI was measured semi-quantitatively: (0) no growth; (1) <10 cfu; (2) 11-50 cfu; (3) semi-confluent growth; (4) confluent growth. 

Results:

Swab type	Study strain	Inoculum (feces-to-AB ratio)	VI by TT			VI by storage condition
			1h	24h	48h	1w		1m
						4°c	-80°c	4°c	-80°c
SS	1	1:1	4	4	4	3	3	0	3
		1:100	1	4	4	1	2	0	2
		1:10-4	1	4	4	0	0	0	1
	2	1:1	4	3	4	3	3	0	1
		1:100	3	4	4	2	2	0	1
		1:10-4	1	2	2	0	0	0	0
ES	1	1:1	4	4	4	3	3	2	3
		1:100	2	4	4	2	1	0	2
		1:10-4	1	4	4	1	4	0	4
	2	1:1	4	4	4	3	1	2	4
		1:100	3	4	4	2	2	0	2
		1:10-4	1	3	4	0	1	0	1

Conclusions:

Both swab types showed good recovery of AB when plated on selective media, even at low inocula and especially if bacterial overgrowth occurred at prolonged TT.  MDR-AB recovery was better with ES following storage, especially with low inocula or unfavorable conditions. ES is thus anticipated to be superior for recovering AB from rectal surveillance specimens, especially in low-level carriage states and if swabs should be stored refrigerated or frozen until processed.