This study aimed at relating the development of clinical infection in dog-to-dog bite wounds with the type of bacterial contamination present, from environmental, oral or skin microbiota sources. The study comprised 48 untreated dog bite wounds, 23 of which clinically infected (48%). Infection identification was based on the presence of pus, fever, and leukocytosis or when 3 or more of the following minor criteria were present: erythema, oedema, subcutaneous emphysema, tissue necrosis, and malodor. Samples, collected before any wound manipulation, with cotton swabs or by needle aspiration, were submitted to bacteriological analysis. All wounds bear aerobic bacteria: from a total of 125 isolates, the most frequent isolates were Pasteurella multocida (16.8%) and Staphylococcus intermedius (14.4%). Strict anaerobes were only isolated from infected wounds. The most common isolates from infected wounds were streptococci (23.2%), Enterobactereaceae (20.3%) and P. multocida (17.4%). Anaerobic cultures were positive in 26% of infected wounds, being Clostridium perfringens present in 3 samples. The most common isolates from non-infected wounds were Gram-negative oxidase-positive bacilli (42.8%), including P. multocida (19.6%), and staphylococci (21.4%). The results suggest that both environmental and animal microbiota contamination occur in dog-to-dog bite wounds. Environmental pathogens may represent an important source of infection. The presence of skin and mouth microbiota of dogs, frequently found in both types of wounds, may also represent an important factor for the development of infection. Further studies are required on microbiota virulence factors, in order to deeply understand their role in the outcome of wound contamination.