## 1 Introduction  
The success of oral rehabilitation in patients undergoing implant treatment largely depends on the health of the tissues. As the oral cavity is a dynamic system, continuously colonised by interacting and proliferating microorganisms, it is extremely important to understand the microbiota and control causal factors before, during, and after implant placement to prevent the development of peri-implant disease [1, 2].  
Oral microbiota differ in everyone and, when in balance, these microorganisms do not cause any harm to the oral structure, a phenomenon known as eubiosis, characterised by a mutual beneficial relationship and a defence mechanism against other species [1]. However, alterations in the host's immune system, pH changes, decreased salivary flow, altered activity of salivary proteins, diet (high carbohydrate consumption), poor oral hygiene,tobacco use, diabetes, prolonged use of oral antibiotics/anticribalis/antiseptics and genetic factors can lead to microbial imbalances. Under these circumstances, virulent and opportunistic microorganisms in dysbiosis can cause periodontal and peri-implant diseases [1; 2; 3].  
In peri-implant disease, the biofilm has been reported to contain significant amounts of Gram-negative bacteria, such as _Porphyromonas gingivalis_, _Treponema danticola_, _Tannerella foryythia_ and _Aggregatibacter actinomycetemontans_, but _Candida_ spp., _E. faecalis_ and _P. aeruginosa_ have also been found in implants with peri-implantitis [4; 5; 6; 7].  
_Candida_ species in the oral cavity are commensal and it has been suggested that the subgingival environment can serve as a refuge, where, under favourable conditions, they can transform into opportunistic pathogens and induce oral diseases [8; 9; 10].  
_P. aeruginosa_ is one of the most common microorganisms in healthcare-associated infections, with high mortality rates, especially in severely ill and immunocompromised patients. It is an opportunistic human pathogen characterised by intrinsic resistance to multiple antimicrobial agents. A recent study found significantly higher levels of _P. aeruginosa_ in oral epithelial cells of individuals with periodontitis compared to individuals with a healthy periodontitis [11]. In addition, [PERSON] et al., 2022, isolated _P. aeruginosa_ in 3 out of 30 cases of peri-implantitis, demonstrating that in addition to its difficult elimination when organised in a biofilm, it also showed resistance to 13 out of 16 antibiotics tested, suggesting that resistance to antimicrobials of opportunistic pathogens in peri-implantitis has led to an urgent need to create alternative treatments to antibiotics for the treatment of these infections [12].  
_E. faecalis_ are facultative anaerobic Gram-positive cocci. They are rarely found in the oral cavity under healthy conditions, but have a high occurrence in failed endodontic treatments, persistent periapical lesions, chronic periodontitis and have also been detected in cases of peri-implantitis [4].  
If peri-implantitis is not diagnosed and treated early, extensive bone loss will occur and compromise implant stability; therefore, microbiological diagnosis is important for a more appropriate and effective treatment.  
In Portugal, as far as we know, there are no studies that have evaluated the prevalence of these opportunistic pathogens in implants and adjacent teeth and the implications both in terms of antibiotic prophylaxis and treatment that the presence of these microorganisms represents.  
The aimed of this study was to detect _Candida_ spp., _E. faecalis_ and _P. aeruginosa_ in the periodontal and peri-implant subgingival plaque with or without disease presence and to seek a correlation between the presence of these microorganisms and demographic data, hygiene habits, endodontic treatment and implant connection type.