Antimicrobial resistance in urinary pathogens among Swedish nursing home residents remains low: a cross-sectional study comparing antimicrobial resistance from 2003 to 2012

Residents of the participating nursing homes, regardless of UTI symptoms were requested to participate. Those accepting participation were included after having met the following inclusion criteria:

The following exclusion criterion was used:

Residents were informed of the study verbally and in writing. Informed approval for participation in the study was collected from decision-capable individuals choosing to participate in the study. However, a considerable number of participants consisted of residents with varying degrees of dementia. If incapable of understanding the information and thereby possessing a reduced decision capability, these residents only participated so long as they did not oppose participation, and under the condition that appointed representative or relatives did not oppose participation after having taken part of the study information.

In addition to collecting the urine sample, the attending nurse made an entry in the study protocol, once for each included resident, whether having ongoing or previous antibiotic treatment within the last month or diabetes mellitus. In 2012, the attending nurse also registered overnight admissions to hospital, any antibiotic treatment during the last six months, and an eventual diagnosis of dementia.

The personnel at the nursing homes were instructed to collect a mid-stream morning sample, or a voided urine specimen with so long a bladder incubation time as possible. Dipstick urinalysis was carried out at the nursing home. The microbiology laboratory was provided information on the outcome of the dipstick urinalysis as well as information on any UTI symptoms from the attending nurse. The urine specimens were cultured at the microbiology laboratory in Borås according to clinical routine procedure. The urine samples were chilled before transport and usually arrived at the laboratory within 24 hours.

The laboratory fractionated 10 μl urine on a cystine-lactose-electrolyte deficient agar (CLED) and a Columbia blood agar. Plates were incubated overnight (minimum 15 h) at 35–37°C. CLED plates were incubated in air and Columbia plates were incubated in 5% CO₂. The latter was further incubated for 24 hours if no growth occurred after the first incubation. Growth of bacteria was considered significant if the number of colony-forming units (CFU)/mL was ≥10⁵. However, at signs of possible UTI such as a positive nitrite dipstick, leukocyte esterase dipstick >1, fever, frequency, urgency or dysuria, the cut-off point was ≥10³ for patients with a growth of Escherichia coli (E. coli) and male patients with Klebsiella species (spp) and Enterococcus faecalis (E. faecalis). For symptomatic women harbouring the two latter species the cut-off level was set at ≥10⁴. At these lower cut-off points and with no symptoms or signs of possible UTI, the urine cultures were classified as sparse growth. For the purpose of this study, both significant and sparse growth were defined as a positive urine culture. In case of several detected species, E. coli was selected in favour of secondary pathogens such as Klebsiella spp. and in case of several E. coli, the most prevalent isolate was selected. Growth of mixed flora was classified as a negative urine culture.

The antimicrobial susceptibility of bacteria was determined according to the disk diffusion method described by the Swedish Reference Group for Antibiotics (SRGA) at the time. In 2012 antimicrobial susceptibility tests followed guidelines and breakpoints proposed by the European Committee on Antimicrobial Susceptibility Testing (EUCAST) for the standardised disk diffusion test [30]. In 2003 nalidixic acid was used as a screening disk for any quinolone resistance; in 2012 isolates were only tested for ciprofloxacin resistance according to national guidelines. Bacterial isolates with suspected extended spectrum beta-lactamase (ESBL) production were confirmed as ESBL-producing bacteria by the reference laboratory at the Swedish Institute for Communicable Disease Control.

Differences between bacterial species and resistance patterns between 2003 and 2012 were analysed by Pearson chi-square, and when appropriate, Fisher’s exact test.

IBM SPSS Statistics version 21 was used for statistical analysis.

In 2003, 751 of 1187 individuals in 32 nursing homes fulfilled the inclusion criteria, and 655 (87%) accepted participation (Figure 1). Voided urine samples were provided from 651 individuals, 482 (74%) women and 169 (26%) men. Women’s ages (mean 86 years, SD 7.4, range 46–102) were slightly higher than men’s (mean 82 years, SD 7.8, range 54–99) (p < 10^-6). 100/651 (15%) suffered from diabetes mellitus. When the urine specimens were collected, 26/651 (4.0%) were undergoing antibiotic treatment. Another 61/651 (9.4%) had no ongoing antibiotic treatment when the urine specimens were collected, but had received antibiotics during the previous month. The antibiotic treatment history was, however, unknown for 12/651 (1.8%). The presence of new or increased urinary symptoms occurring during the previous week was; for dysuria 5/651 (0.77%) and urinary urgency 6/651 (0.92%).

In 2012, 735 of 901 individuals in 22 nursing homes fulfilled the inclusion criteria, with 484 (66%) accepting participation (Figure 2). Urine samples were provided from 480 individuals in 2012 (421 voided urine samples and 59 urine samples from indwelling urinary catheters); 321 (67%) women and 159 (33%) men. Women’s ages (87 years, SD 6.6, range 62–100) were slightly higher than men’s (mean 85 years, SD 7.0, range 65–100) (p = 0.010). 71/480 (15%) suffered from diabetes mellitus, and 253/480 (53%) had dementia. When the urine specimens were collected, 23/480 (4.8%) had ongoing antibiotic treatment. Another 45/480 (9.4%) had no ongoing antibiotic treatment when the urine specimens were collected, but had received antibiotics during the previous month. The presence of new or increased urinary symptoms occurring during the last week was for dysuria 5/480 (1.0%), urinary urgency 6/480 (1.3%), and urinary frequency 2/480 (0.42%).

Men’s mean age was 3.1 years higher in 2012 compared to 2003 (p = 0.00016), while there was a statistically insignificant trend towards a higher mean age among women (0.92 year, p = 0.073). The proportion of men was higher in 2012; 33% compared to 26% in 2003 (p = 0.0087).

E. coli was by far the most common bacterial species. Bacterial growths in urine cultures and distribution among bacterial species are presented in Tables 1 and 2.

The proportion of positive urine cultures from voided urine was more common among women than men; 184/482 (38%) versus 23/169 (14%), p < 10^-6 in 2003 and 134/295 (45%) versus 13/126 (10%), p < 10^-6 in 2012.

The average resistance rates for all tested antibiotics were similar between 2003 and 2012, however there was a non-significant trend (p = 0.090) towards higher resistance rates for cefadroxil in 2012, but still at the low level of 2.6% for all E. coli isolates (Table 3). It was not possible to compare resistance rates for quinolones between 2003 and 2012 as nalidixic acid was no longer used as a screening disk for quinolone resistance in 2012.

In 2003, no ESBL-producing Enterobacteriaceae were found.

Predictors for antibiotic resistance in voided urine specimens are presented in Table 4.

The resistance rates among isolated Klebsiella spp. in voided urine in 2012 were; for ampicillin 91% (10/11), nitrofurantoin 91% (10/11), mecillinam 0% (0/11), ciprofloxacin 0% (0/11), trimethoprim 27% (3/11) and cefadroxil 0% (0/11). Similar rates were seen in 10 Klebsiella isolates from indwelling urinary catheters.

In this study 46% (27/59) of the cultures of urine specimens obtained from indwelling urinary catheters were classified as positive. There were growths of mixed bacterial flora in all but one of the cultures obtained from urinary catheters, classified as negative. The bacterial findings in positive urine cultures obtained from urinary catheters were; E. coli 48% (13/27), Klebsiella spp. 37% (10/27), Proteus mirabilis 11% (3/27) and Enterobacter spp. 3.7% (1/27).

Resistance rates in E. coli (catheter urine specimens) in 2012 were; for ampicillin 46% (6/13), trimethoprim 15% (2/13), mecillinam 0% (0/13), ciprofloxacin 15% (2/13), cefadroxil 7.7% (1/13) and nitrofurantoin 0% (0/13). There was a trend towards higher resistance rates in E. coli in urine specimens from catheters compared to voided urine for ampicillin (p = 0.079) and ciprofloxacin (p = 0.11).

There were still comparatively low levels of antimicrobial resistance in urinary pathogens among Swedish nursing home residents with no major changes between 2003 and 2012. Any antibiotic treatment during the last month and hospitalisation during the last six months predicted higher resistance rates among urinary pathogens.

A major strength of this study is that urine samples were collected from all those capable of providing a urine sample at the participating nursing homes for the elderly. Thus, this study describes the native antimicrobial resistance in nursing homes for the elderly. Previously, most studies of antimicrobial resistance in uropathogens among elderly residents of nursing homes compiled antimicrobial resistance in urine cultures taken when the clinician suspected a UTI. Those with UTI symptoms and underlying diseases in the urinary tract comprised a selected patient group assumed to have higher resistance rates.

Another major strength is the possibility to compare resistance rates at nursing homes in the same geographical area between 2003 and 2012. Furthermore, since this study includes many nursing homes they can be assumed to be representative of all Swedish nursing homes, especially whereby nursing homes are similarly organised in all regions of Sweden (municipal care).

In this study we obtained urine specimens and study protocols from 55% (651/1187) of the individuals registered at the nursing homes in 2003, and 53% (480/901) in 2012. This may appear low but is similar to previously published studies in nursing homes for the elderly [31]. One major reason for not participating in this study was substantial urinary incontinence, often combined with dementia. 8.1% (96/1187) and 25% (222/901) of the individuals registered at the nursing homes refused participation in 2003 and 2012, respectively. Still this may be considered acceptable when studying an elderly fragile population with a high proportion of residents with dementia as well as the ethical requirement of approval from appointed representatives/relatives.

Several studies have shown increased antimicrobial resistance in uropathogens during the last ten years [2, 32]. In this study the average resistance rates for all tested antibiotics were similar between 2003 and 2012. In contrast to most countries the prevalence of ESBL-producing Enterobacteriaceae was low in this study. This may be partly due to successful efforts in Sweden to lower antibiotics usage and the choice of narrow spectrum antibiotics in favour of e.g. ciprofloxacin [23, 33].

The overall resistance rates in E. coli for UTI antibiotics in Sweden in 2012 [23] as compared to this study (% R in Sweden/% R in this study) were; for ampicillin 31%/21%, mecillinam 4.6%/7.7%, cefadroxil 3.5%/2.6%, nitrofurantoin 1.1%/0.85%, trimethoprim 19%/12% and ciprofloxacin 7.6%/3.4%. Thus, the proportion of resistant bacteria was lower than expected for all antibiotics excepting mecillinam in this study. This might be partly explained by national statistics being based on clinical isolates and our material from all those able to provide a urine specimen. On the other hand a nursing home population can be assumed to have higher antimicrobial resistance rates since they have a high level of co-morbidity and are prescribed more antibiotics than younger individuals. Thus, it is satisfying to note that a Swedish nursing home population still has comparatively low antimicrobial resistance rates among E. coli.

Hospitalisation during the previous six months increased the risk for antibiotic resistance in E. coli against ampicillin, ciprofloxacin and any antimicrobial tested, adjusted for age, gender and any antibiotic treatment during the previous six months. This is consistent with several articles reporting prior hospitalisation as a risk factor for antimicrobial resistance which suggests that the hospital is a source of antibiotic-resistant organisms [15].

There was a low prevalence of new or increased dysuria and urgency in this study compared to studies collecting urine specimens from patients with suspected UTI. This was expected as it was a screening study collecting urine specimens from all those able to provide a urine sample at the participating nursing homes, regardless of the presence of symptoms.

Procedures utilizing the presence of symptoms or outcomes of prior dipstick testing, to influence the setting of cut-off levels for CFU/mL in urine cultures to label growth as clinically significant, may enhance the diagnostic procedure [34]. This procedure has also been used in previous studies of antimicrobial susceptibility whereby it reflects the clinical situation [6, 32, 35]. Few individuals had any specific symptom of UTI, but many had significant growth, which is an important result to consider when evaluating urine samples in this population. In this study, when analysing antibiotic susceptibility all isolates with sparse or significant growth were included to identify the pool of resistant bacteria in the nursing home population.

In general, there are some minor differences in breakpoints between 2003 and 2012 due to methodological changes over time. In case of a difference, the breakpoints have only been changed so that a bacterium is more likely to be classified as resistant in 2012 as compared to 2003. For the quinolones, nalidixic acid was no longer used as a screening disk in 2012. Thus, it is not possible to properly compare resistance rates for quinolones during the studied period.

It is inappropriate to compare resistance rates for Klebsiella isolates between 2003 and 2012 due to the low numbers of Klebsiella spp. in 2012.

E. coli was by far the most common finding among all isolated species both in 2003 and 2012. However, the proportion of E. coli in voided urine was higher in 2012; 80% (117/147) as compared to 69% (143/207) in 2003. This could in part depend on the lower proportion of E. faecalis in 2012; 0.68% (1/147) versus 8.2% (17/207) in 2003. Even if these are small numerical differences, we cannot provide any satisfactory explanation. We have no data indicating more complicating factors within the urinary tract in 2003, since we have not investigated for that.

Urine culture screening for bacteriuria was used in this study to describe native antimicrobial resistance, however, in clinical practice urine specimens should not be considered unless patients have symptoms from the urinary tract.