Numerous studies presented divergent points of view on how bacterial contamination can be observed in different locations within hospital facilities, how seasonal variations affect the concentration, and even how data regarding the most prevalent genera that can be found in healthcare environments and staff due to occupational activities.

In the first presented study [34], concentrations of airborne bacteria reached a peak in November (436.3 CFU/m³), and the lowest value was identified in December (257.1 CFU/m³). However, the authors did not explain the decrease between these two months or between the other months of the year that were assessed. Another study, conducted in Portugal [45], described that most of the departments analyzed presented a high level of bacterial concentration (ranging from 12 to 170 CFU/m³) and concluded that the indoor bacterial concentration was not influenced by outdoor concentration but by indoor air.

In another analyzed study [37], winter and autumn had the highest concentration levels of bacteria with the increased bioburden associated with the waiting room in autumn and children’s ward in the winter. A possible explanation for the higher load in the waiting room can be attributed to an increase in the number of occupants (200 per day or more). The same hypothesis is suggested in the children’s ward, with the additional factor of the increased number of infections in children during the wintertime. The fact that the rooms in that ward are small, and that more than one parent can accompany the child, is another factor that increases the concentration of bacteria such as airborne actinomycetes, which were the most prevalent in the children’s ward [37].

The data from Portuguese studies revealed that Staphylococcus epidermidis [45] and Micrococcus spp. [45,46,47] were the most prevalent bacteria identified. However, S. epidermidis is not considered to be a relevant risk of exposure due to its lower infections rates [34]. On the other hand, Acinetobacter (species not identified) was found in two studies [32,34] as a prevalent genus in a university hospital from Ethiopia, justifying its persistence due to its great survival ability in the indoor environment [47], which can be a potential cause for hospital infections transmitted via the air [48]. Another relevant microorganism is Pantoea agglomerants, which is able to infect hospitalized individuals, particularly immunodeficient patients, through contaminated medical instruments [49].

A bacteria genus described in several studies considered of great interest is Staphylococcus spp., which was identified at the highest concentration in a cardiology ward/intensive care room (hospital 5). Once the strains were analyzed, S. saprophyticus was the most predominant, followed by S. warneri, being reported as an important opportunistic pathogen and associated with healthcare-acquired infections [37]. Cabo Verde et al. [45] also found S. warneri to be a prevalent species in the hospital, but no data were reported regarding S. saprophyticus in contrast to the findings of Sivagnanasundaram et al. [50]. S. saprophyticus can infect an individual’s blood through catheters, surgical prostheses, pneumonia, urinary tract infections and more [51,52,53]; therefore, this species is of particular relevance. Moreover, Staphylococcus aureus is one of the most relevant nosocomial pathogens, described as one of the main pathogenic bacteria that cause osteomyelitis (including in children)[54,55,56,57,58,59] septic arthritis [60,61,62,63], and prosthetic joint infections [64,65,66,67]. S. aureus is a commensal bacterium and a human pathogen; about 30% of all humans are colonized with S. aureus [68]. Osteoarticular infections can occur by the direct inoculation of microorganisms into tissues due to penetrating or open trauma [69]. In limited settings, infections complicate as many as 44% of open fractures [69]. Trauma-related osteoarticular infections among patients with punctures wounds seeking medical care varied from 2–60% depending on the type of the injury [70,71,72]. S. aureus was also prevalent in the studies by Matinyi et al. [38] and Thomas et al. [73], being detected on door handles, which could be considered a possible source of nosocomial infection [38]. Additionally, Staphylococcus epidermidis, as well as Micrococcus luteus and Bacillus subtilis, were common in the studies by Sivagnanasundaram et al. [50], Bielawska-Drózd et al. [74], and Bolookat et al. [39]. The reason for these results could be related to the presence of these bacteria in the skin, mucous membranes, hair of human beings and animals [75,76,77,78].

Considering health effects, the identification of Clostridium difficile, a Gram-positive, anaerobic, spore-forming bacillus, must be considered, as it is the principal agent of pseudomembranous colitis in humans [79]. In the hospital setting, C. difficile infection is the main cause of healthcare-associated diarrhea [80]. Symptoms arise when C. difficile spores germinate in the intestine, and the bacteria start to produce toxins, Toxin A (TcdA) and Toxin B (TcdB), causing the inflammation of the large intestine [81]. The clinical presentation can range from mild diarrhea to lethal toxic megacolon [81]. Nonetheless, the ingestion of C. difficile spores does not always lead to the development of symptomatic disease, since this bacterium can be silently present in the intestine without manifesting any symptoms, denominated as asymptomatic C. difficile colonization [82]. However, the patients with this condition act as a reservoir for further transmissions [83,84], and they can progress to the infection themselves, especially if they are affected by an underlying illness [85]. The infection is transmitted by spores that are environmentally resistant [86].

Another microorganism of interest is H. pylori, which is associated with occupational exposure among endoscopy personnel [87]. The study by Mastromarino et al. [40] aimed to determine if different staff groups of healthcare workers, either with or without direct patient contact, were at equal risk of acquiring H. pylori infection. The authors concluded that direct contact with patients is an important factor for becoming infected, as opposed to simply working in the endoscopy unit. Another study highlighted [87] that 24% (9 out of 37) of gastrointestinal endoscopy personnel, and 47% (33 out of 70) of workers in a hospital who care for disabled individuals, tested positive for H. pylori. In this case, direct contact with patients and working in a hospital where disabled individuals reside was associated with H. pylori infections, but the exposure to gastrointestinal secretions of the patients was not [87]. This study supports the previous study’s idea that direct contact with patients is a factor to take into consideration.

Among Gram-negative bacteria, the most frequently identified pathogens are Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii. E. coli was described to be prevalent in operating beds due to fecal contamination and the lack of efficiency in the cleaning process [88,89]. Pseudomonas spp., one of the most significant genera of nosocomial pathogens, was the most prevalent bacteria in all operating theatres in the Matinyi et al. study [38], and was identified in other studies [73,90] that referred to antiseptic solutions as a possible source of this contamination [56,57,91,92]. Other bacteria, Bacillus spp., can be related to dusty environments, the type commonly found in Uganda [38]. This contamination might occur in indoor areas if the windows are open for natural ventilation and could then be transferred to the operating theatres [73]. Furthermore, A. baumannii was associated with a vast number of infections, such as those of the bloodstream, respiratory tract, surgical sites, and urinary tract [93]. Transmission between patients in hospital settings is difficult to prevent because of the bacterium’s capacity to persist in the environment, particularly in intensive care units [93]. These Gram-negative pathogens are associated with the etiology of numerous and severe hospital-acquired infections in humans and have the capacity to resist antimicrobial agents, which has become an increasingly relevant problem [94]. Currently, antimicrobial-resistant Gram-negative bacteria are expanding worldwide and are considered a significant threat to human health [3].