Increased resistance of microorganisms to antibiotics and disinfectants and emergence of biocide-resistant bacterial strains have made it difficult to eliminate them. Disinfectants are effective in reducing health care associated infections, but the correct choice of disinfectants considering the wide range of efficacy against hospital pathogens is important and essential in this process [15, 18]. The purpose of this study was to evaluate the efficacy of three hospital disinfectants in two strains of pathogenic bacteria based on MIC and MBC tests. According to Tables 2, 3, and Fig. 2 the disinfectants did not have an equal efficacy in the tested strains and B. cepacia showed more resistance to all tested disinfectants compared to E.faecalis. It can be concluded that the same disinfectants should not be used for all wards, sections and places in healthcare settings. The results of this study showed that under the same conditions, different disinfectants revealed various efficacies and diverse ranges of activity against different bacterial strains. On the other hand, equal concentrations of disinfectants in the same application situation may have different effects on different types of bacteria. Therefore, before choosing and applying any disinfection in hospitals, it is absolutely necessary to measure the MIC and MBC values of daily used disinfectants against common life-threatening bacterial agents or healthcare associated infections.
MIC and MBC assay tests are tools to determine the effectiveness of disinfectants against micro-organisms and to determine the sensitivity of bacteria in health care settings [19, 20]. If MIC and MBC values of a disinfectant are small enough, bacterial growth is inhibited at low concentrations of the disinfectant; in this case, the disinfectant is stronger, more effective, and suitable. In this situation, if the disinfectatnt is inexpensive (reasonable price), it is considered the best one compared to others. MIC values are usually less than MBCs, because higher concentrations of an antibacterial agent are needed for bactericidal versus bacteriostatic effects in most cases. MIC is the lowest concertation that inhibits the growth of tested bacteria, whereas MBC is usually a relatively higher concentration of the disinfectant that kills the microorganism. According to our findings, MIC and MBC of the two disinfectants were equal values in all experiments; this could be due to the optimal antibacterial properties of these biocidal agents. The MIC and MBC results of MQ showed that in the same conditions, it has different antibacterial effects on tested strains. MQ, at a concentration of MIC = MBC = 70.31 mg/L, reduced the growth of B. cepacia ATCC 10673 to 3-Log (99.9%), whereas a 3-Log reduction (99.9%) in growth occurred at MIC = MBC = 2.2 mg/L for E. faecalis ATCC 29212. Therefore, it can be concluded that either E. faecalis ATCC 29212 is more susceptible than B. cepacia ATCC 10673 to this disinfectant or the antibacterial effect of MQ is weaker on the B. cepacia ATCC 10673. Lower sensitivity of gram-negative versus gram-positive bacteria is mostly attributed to differences in chemical composition of gram-negative and gram positive cell walls, resulting in different bacteria behaviors against disinfectants. The cell wall of the gram-negative bacteria contains an outer membrane  that partly prevents the penetration of disinfectants and antibiotics into the bacterium. Acquisition of certain types of plasmids can make gram negative bacteria more resistant to biocides because of the production of detoxifying enzymes . In this study, B. cepacia ATCC 10673 (gram negative) was more resistant to disinfectants compared to E. faecalis ATCC 29212 (gram-positive), but the mechanism of resistance was not investigated. Therfore, more detailed studies are required to explore the mechanism of resistance of the studied bacteria to these disinfectants.
E.faecalis showed a high sensitivity to Steranios 2% (MIC = MBC = 0.31 mg/L). However, in the same conditions, the lowest bacteriostatic and bactericidal concentration of Steranios 2% for B. cepacia was 9.83 mg/L. It was also observed that E.faecalis was less resistant to lower concentrations of disinfectant. So, in hospital wards where E.faecalis is prevalent, even low concentrations of Steranios 2% can be applied for eradication of this strain. This recommendation should be taken with cautionbecause the rate of resistance to Steranios 2% in other prevalent hospital-acquired pathogens should be determined and considered as well. The major components of Steranios 2% are glutaraldehydes, which are used as high-level disinfectants in healthcare settings for sterilizing medical instruments such as endoscopes, lenses, rubber, plastic, etc. Glutaraldehyde has no corrosive properties and its shelf- life is 14 days . It stimulates the skin, eyes, and respiratory tract and there are occasional health problems such as occupational asthma in dealing with glutaraldehyde . Therefore, when glutaraldehyde is used as a disinfectant, care should be taken and attention should be paid to ventilation. The range of bactericidal (MIC) or bacteriostatic concentrations (MIC) of Steranios 2% was 5035–9.83 mg/L for B. cepacia and 5035–0.31 mg/L for E.faecalis. Pariscila et al. , showed that the MIC values of glutaraldehyde for E. cloacae, B. subtilis and E. coli were in the range of 2750–3750 mg/L. Whereas, our results show that the MIC values were markedly lower in the present study (0.31 mg/L vs. 2750–3750 mg/L).
Another disinfectant in this study was DH whose effectiveness was compared with other tested disinfectants. Its main ingredient is peracetic acid (PAA). Peracetic acid is a suitable alternative for decontamination of heat-sensitive medical equipment. This antibacterial agent has a rapid activity against bacterial spores, but it has corrosive effects on some metal equipment . The present study also showed (Table 3) that B. cepacia was generally less sensitive to DH than E. faecalis. At 19.53 mg/L, DH showed marked antibacterial effects against E. faecalis whereas B. cepacia was more tolerant to it. The bactericidal concentration range of DH was 5000–156.25 mg/L for B. cepacia and 5000–19.53 mg/L for E. faecalis. So, it can be concluded that 19.53 mg/L of DH has bactericidal effects on 99.9% of E. faecalis population, but the same concentration is ineffective on B. cepacia. In another study, the MIC range of peracetic acid for B. cepacia and some gram-negative strains was 2310–4491 mg/L  which was in the scope of our study. The median concentration of Sterniose 2% against E. faecalis and B. cepacia was lower than DH and MQ, respectively (Fig. 2). In addition E. faecalis needs a lower bactericidal concentration range of disinfectants compared to B. cepacia and a lower median of a disinfectant is the most effective against pathogens.
MIC and MBC represent the minimum inhibitory and minimum bactericidal concentration of disinfectants at 24 h , However, to minimize the risk of infection in hospital environments, there is a need for a potent antibacterial activity in very shorter exposure time for disinfection of critical equipment. Therefore, future studies should be conducted to determine the bacteriostatic and bactericidal effects of disinfectants at short time exposures, including 5, 10, and 20 min. Due to the limited number of critical items (e.g. endoscopic instruments, bronchoscopes, arthroscopy devices, etc.) in our hospital and the physicians’ interest in visiting more patients in less time, it is recommended to determine the effective concentrations of various disinfectants (low, intermediate, and high level) in short contact times with pathogenic microorganisms. In this study, the bacteriostatic and bactericidal effects of three different disinfectants were investigated on reference isolates of E. faecalis and B. cepacia. It is obvious that bacterial isolates from patients, hospital personnel, or hospital environment are usually more resistant to antimicrobial agents, so further studies in a larger number of hospital isolates are recommended. The present study showed the different efficacy of three classes of disinfectants on two different strains of bacteria. Considering the variable effects of disinfectants on several nosocomial pathogens, alternate use of disinfectants in hospital wards is highly recommended. These findings could be used in hospitals or health care settings where the efficacy of disinfectants is important.