What is new in selective decontamination of the digestive tract?

Selective decontamination of the digestive tract (SDD) and selective oropharyngeal decontamination (SOD) are among the few interventions in intensive care medicine that have been shown to improve patient survival, but their use is limited to a minority of European intensive care units (ICUs) (Tables 1, 2) [1].

Table 1 Large studies comparing SDD and SOD

Table 2 Post hoc analyses and secondary studies on SDD and SOD

In addition, when the microbiological data of patients receiving SDD or SOD were compared with those receiving standard care, ICU-acquired bacteremia was significantly reduced for Staphylococcus aureus, glucose-non-fermenting Gram-negative rods, and Enterobacteriaceae [4], In particular, the use of SDD was associated with a lower incidence of acquired bacteremia with Enterobacteriaceae. Similarly, ICU-acquired candidemia was lower in the SDD group than in the SOD group or standard care group, but the difference was not significant. These findings were confirmed in a recent study comparing SDD and SOD on antibiotic resistance. The incidence of ICU-acquired bacteremia was also lower for aminoglycoside-resistant Gram-negative bacteria in the SDD group [5]. Although the survival rate of ICU patients remains similar in both studies, the lower incidence of antibiotic resistance and nosocomial bacteremia as consistent findings are in favour of SDD.

Common reasons for the reluctance to use SDD or SOD are related to only a few arguments regularly mentioned in editorials and by expert opinion expressing the fear that their use may promote antibiotic resistance and the possible increase of methicillin-resistant S. aureus [15]. These can be summarized as follows:

• 1. The absence of emergence of resistance is against current microbiological concepts and contradicts the worldwide pandemic of multidrug-resistant microorganisms demonstrated to be directly related to the use of antibiotics. In a recent meta-analysis, no relation was observed between the use of SDD and the development of antimicrobial resistance, thus confirming earlier reports [16]. Recent studies have demonstrated similar findings (Table 2). In a large study showing lower mortality with the use of SDD or SOD compared with standard care, patients treated with SDD and SOD had a significantly lower incidence of carriage and infections with antibiotic-resistant bacteria [4]. Moreover, when compared with SOD, SDD was related with lower rectal carriage of antibiotic-resistant Gram-negative bacteria [5]. By contrast, the continuous application of antibiotics included in the paste, as well as the aerosolized colistin applied in the case of emergence of Gram-negative bacilli in the respiratory samples, may largely contribute to the absence of the documented emergence of resistance (footnote Table 1).

• 2. One of the main reasons of bacterial resistance to antibiotics is the widespread use of antimicrobial agents. This represents the main reluctance for the use of SDD. Surprisingly, some investigators have even advocated for the use of SOD due to the absence of widespread systemic prophylaxis with cephalosporins and a lower volume of topical antibiotics [4]. Indeed, when SDD was compared with standard care, the use of cephalosporins was increased due to the SDD regimen, but the use of antimicrobial agents was reduced significantly for broad-spectrum penicillins, carbapenems, lincosamides, and quinolones [4]. This was also true for SOD, but the difference with standard care was less pronounced [4].

• 3. Recent SDD/SOD studies were all performed in the Netherlands where antimicrobial resistance is a minor concern with a low reported use of broad-spectrum antibiotics, such as piperacillin/tazobactam, cefepime, and carbapenems. Hence, a more pronounced gradual increase was observed with aminoglycoside-resistant Gram-negative bacteria with SDD [5]. The effects of the prolonged use of SDD and SOD on colistin resistance have been determined in a study performed on two different large ICU cohorts [13]. No association was observed between the use of SDD or SOD and increased acquisition of colistin-resistant Gram-negative bacteria in the respiratory tract. In another study performed on patients colonized with Enterobacteriaceae in the intestinal tract at ICU admission, SDD was shown to eradicate cephalosporin-resistant Enterobacteriaceae from the intestinal tract [11]. These findings are usually related to the fact that the studies are performed in environments with a lower incidence of highly-resistant microorganisms. By contrast, studies performed in countries with a higher incidence of highly-resistant microorganisms have also reported similar effects [17, 18].

• 4. Some observations were performed over a short period of time and resistance may not have been immediately apparent. Hence, a rebound effect after stopping SDD/SOD has been suggested in one of the post hoc analyses, as well as the emergence of colistin-resistant strains during persistent Gram-negative bacteria colonization over the study period (24 months) [13, 7]. Indirect evidence suggests that SDD/SOD is associated with the long-term alteration of the microbiota of the digestive tract and a potential increase in the associated resistome, but this remains largely speculative at the present time [19]. However, these effects were not confirmed in a very recent report on continuous surveillance of the impact of SDD and SOD up to 7 years [14]. This large study confirmed a continuous reduction of the rate of tobramycin resistance and the absence of emergence of resistance to colistin in both respiratory and rectal samples (Table 2). The occurrence of a rebound effect after the discontinuation of SDD/SOD use in these centres remains to be determined.

In conclusion, SDD and SOD are used in a minority of ICUs, despite the available data on survival benefit. Although antibiotic resistance is not shown to be associated with the use of SDD and SOD in the particular setting of experienced Dutch ICUs, some ecological changes in ICUs have been reported (Table 2). SDD has resulted in lower rectal carriage of antibiotic-resistant Gram-negative bacteria compared to SOD. SDD has demonstrated superiority over SOD, but both are related to a lower use of systemic antibiotics, other than those used during the first 4 days of SDD, and result in a lower mortality in ICU patients compared to standard care. Therefore, SOD can be viewed as a good alternative to SDD. However, the lower rate of bacteremia and bacterial resistance observed with SDD pleads in favor of this regimen. Further studies are planned in higher endemic resistance regions to assess the effect of SDD or SOD on long-term resistance development.