Sampling and culture of microorganisms
The microbial concentration in an indoor OR is one of the extrinsically influencing factors for SSI. Despite our results demonstrated that free fungi and the number of mean bacterial colonies were far from the suggested levels during surgical activity despite a well-controlled OR ventilation system with a HEPA filter.
Inadequately filtered incoming air was the major source of fungal contamination; the suggested alert values were 4 cfu/m
3 [
20]. Most reports suggest that an acceptable bacterial limit for a working OR is below 180 cfu/m
3 [
6,
10,
16,
21]. However, differences in institutions, OR settings, sampling methods, study periods and sampling locations might account for some differences in microbial concentrations between studies. Pasquarella et al. performed microbial monitoring in 29 conventionally ventilated ORs over a three-year period; they found that bacterial contamination values in working ORs varied widely. The mean bacterial contamination was from 122 to 149.7 cfu/m
3; however, 6%–27% of the samples had more than 180 cfu/m
3 and maximum values of 798 cfu/m
3 [
10]. Another report found airborne bacterial counts ranging from 87 to 585 cfu/m
3 [
11]. In our study, the mean colony counts obtained in ORs were 78 ± 47 cfu/m
3 (with the lowest counts in general surgery of 67 cfu/m
3 to the highest in transplant surgery of 123 cfu/m
3), which was much lower than 180 cfu/m
3. The data of active sampling are known to be highly variable, and its possible influencing factors include the absorption of the medium, the amount of air drawn, and the size-selection sampling head designed [
22]. We used the one-stage Andersen sampler to collect data on microbiological counts, unlike the sampling collection instruments used in previous study [
10,
11]. In the recommendations of the guideline for prevention of SSI, conventional OR ventilation systems produce a minimum of about 15 ACH, three (20%) of which must be fresh air [
5]. Several studies reported that air conditioning frequencies were set at 15 ACH in their ORs [
10,
15,
16]. In comparison, the ORs of this study were taken with a higher standard air conditioning setting (40 ACH and 100% fresh air), which may have caused microbial colony counts lower than some of the other reports.
Our data also found that 4.4% (n = 11 of 250) of the samples had colony counts over 180 cfu/m3; of them, 82% (n = 9 of 11) of samples were taken before incision staging. Personnel movements were more frequent during the patients’ preoperative preparation period, such as laying surgical drapes. These behaviors may lead to an increase in the number of bacteria in the air.
Several reports noted that gram-positive bacteria, including
Staphylococcus spp.,
Bacillus spp., and
Micrococcus spp., often existed in the OR area, followed by gram-negative bacteria, such as
Acinetobacter spp.,
Moraxella spp.,
Pseudomonas spp., and
Stenotrophomonas spp. [
15,
19,
23]. Of these bacteria,
S. aureus was highest in all locations in the ORs and showed resistance to methicillin and ampicillin [
5,
23,
24]. Because a disrupted skin barrier promotes skin colonization by microbes, these investigations detected pathogenic bacteria, such as
Staphylococcus spp.,
Acinetobacter spp., and
Pseudomonas spp., which are implicated in healthcare-associated infections in ORs [
25]. These bacteria also cause outbreaks of SSIs [
5]. Similar to previous reports, our data sampled from the ORs most frequently contained isolates of
staphylococci (5.6%). In recent years, concern has been expressed about the risks posed by contaminated mechanical ventilation ductwork in hospital buildings. The role of environmental contamination in the spread of Gram-negative bacterial infections is being increasingly recognized [
20].
Factors influencing microbial colonies
The number of microbial colonies in the OR was influenced by a variety of causes; the main factors could be divided into indoor environmental- and personnel-related factors [
1,
6,
15,
23]. For intraoperative characteristics, we found that the surgical procedure had a statistically significant effect (
p = 0.012) on microbial colony counts. Of these procedures, the highest mean numbers of colonies were found during transplant surgery and pediatric surgery. The reasons maybe that transplant surgeries require a relatively high number of staff to participate in the operation, while more people enter into the OR during pediatric surgeries, such as interns and parents who accompany the child before anesthesia. Our results are similar to a previous report, that the mean colony counts during pediatrics surgery were the second highest of all surgeries [
10]. We also used multiple regression to further control for covariance; the mean bacterial colony counts in pediatric surgery were significantly higher than in adult surgery (
p = 0.041).
In addition, the site of procedure had a significantly positive correlation (r = 0.164, p = 0.005) with bacterial counts in the ORs; deep incision sites and organ/space sites had higher counts than superficial incision sites with mean bacterial counts of 20.19 cfu/m3 (p = 0.041). This may be because deep incision site and organ/space site surgery requires sewing subcutaneous tissue, fascia and muscle layers, so these operations are more difficult and complex, requiring more assistant physicians.
Several reports noted the number of staff and their activities also influence microorganism concentrations in the air of ORs [
1,
15‐
17,
23]. The microbial level in OR air is directly proportional to the number of people moving about in the room [
5,
18]. A study evaluated long-term variation in air quality in ORs. Their results indicated that the number of people and bacterial concentration were positively correlated (
r = 0.36,
p < 0.01). However, no significant correlation existed between the number of people in a space and the airborne bacterial concentration after adjusting for temperature, RH, and sampling location [
23]. In the present study, the mean number of indoor staff was six people; the results also showed a significantly positive correlation (
r = 0.217,
p < 0.001) between the number of indoor staff and bacterial concentration. Even after adjusting for other potential confounders by multiple regression analysis, these two variables were still significantly correlated, with an increase in indoor colony counts with the addition of each person. Thus, the number of people entering the OR should be limited to necessary personnel [
5,
7]. Hospitals should consider controlling the number of occupants (estimated maximum occupancy of 20 persons/1000 square feet) and increasing outdoor air requirements (15 cubic feet per minute/person) in OR areas to achieve acceptable indoor air quality. Further suggestions include using occupancy sensors for selected occupancy types for ventilation control [
9].
Moreover, inferences can be derived from the three reports on the impact of personnel on the number of microbe colonies. Edmiston et al. recovered microorganisms in an empty OR and during vascular surgery using an impactor sampling device with a low microbial threshold; the total microbe concentration increased from fourfold to twelvefold in the presence of OR personnel [
4]. Another report also revealed median bacterial counts of 80 cfu/m
3 in working ORs, significantly (
p < 0.001) higher than the values of 12 cfu/m
3 in empty ORs [
10]. Recently, Dai et al. conducted a study with microbiologic air sampling every 30 min from 7:00 AM-6 PM; the bacterial count results were the highest during preparation periods (during which the medical staff conducted preoperative procedures and postoperative cleaning) with a mean value of 377 cfu/m
3 and were the lowest under static conditions (no medical activity), with a mean value of 162 cfu/m
3 [
11].
Our results were similar to previous studies demonstrating that the microbial colony counts had the greatest degree of correlation (r = 0.346, p < 0.001) with the surgical stage in the regression model; counts sampled before the incision stage and after the wound closure stage were higher than those during incision to wound closure stage with colony values of 36.5 cfu/m3; four-fifths of the microbial colonies detected over the threshold were also at the before incision stage. Our study also showed that the preparation period had the most frequent personnel activity, including door opening, covering and removing cloth sheets and other items, causing air fluctuations and raising dust in the process.
It may be these reasons that in the regression model after controlling for other variables, the affect of surgery time (during incision to wound closure) on bacterial counts in ORs was a negative regression coefficient. In addition, the ventilation systems are running continuously during this period. It had no statistically significant differences, and the 95% CI (−6.25–2.80) included positive regression coefficients.
Although turbulent ventilation was not related to wound contamination (
p = 0.22), it was associated with an increased number of air microbial counts (
p < 0.001) [
26]. Air in the OR may contain microbial-laden dust, lint, skin squames, or respiratory droplets, so microorganisms can collect on OR environment surfaces [
2,
15,
20,
24]. Thus, conventional cleaning is also necessary.
Opening the door of the OR was not only one of the causes of fluctuations in indoor air but also affects the OR air ventilation so that it cannot completely remove contaminants from the air [
4]. We found a significant positive correlation (
r = 0.109,
p = 0.043) between the frequency of opening the door of the OR and bacterial counts. Doors should be kept closed in surgical procedures except as needed.
Additionally, the number of people in the OR was found to be significantly positively correlated with the air temperature [
15]. Temperature and humidity influence viral, bacterial and fungal particles [
3,
27]. Therefore, environmental factors substantially influence the efficacy of airborne disease transmission [
3,
28]. The present study was similar to a previous report that found no significant correlation between RH and bacterial concentrations [
15]. Although the indoor temperature was not significantly related to the number of microbial colonies, the colony count significantly increased by 9.4 cfu/m
3 with each additional 1 °C (
p = 0.018).
This study design could reflect the actual microbial concentrations during surgical procedures. Several potential limitations of our study should be noted. First, this study was not performed at multiple locations, only in a single medical center with 28 ORs for indoor microbial sampling and data collection, so inferences to other levels of hospitals should be drawn with caution. However, the advantage of this situation was that the settings and equipment of the each OR environment and the experience of the personnel were similar for all working ORs and surgical procedures; this also reduced the bias of the assessment-influencing factors. Second, we excluded bacterial air sampling and data collection on holidays. Final, our purposes of the study were to assess microbial colony counts in working ORs and to determine the factors influencing air contamination. Thus, only bacterial genera were identified. Our study did not detect the bacterial species and perform the molecular typing. We suggest that future studies can add molecular identification, especially in a study of the relationship between microbes in the air and SSIs.