In resource-limited settings, fast and simple point-of-need tests should facilitate healthcare providers the identification of pathogens avoiding empirical suboptimal treatments with broad-spectrum antibiotics. A rapid optical whole cell bacterial biosensor has been here developed using sialic acid functionalized gold nanoparticles allowing the selective screening of Gram-positive Staphylococcus aureus ATCC 25923 and Methicillin Resistant Staphylococcus aureus (MRSA) USA300 and Gram-negative bacteria (Pseudomonas aeruginosa ATCC 15442) by selecting the appropriate dispersing media. Those bacteria were selected due to their common presence in wound bed tissue of chronic infected topical wounds. The discrimination of bacterial pathogens has been attempted in different media including water, two independent buffers, bacterial broth, human serum and human urine. The identification of Gram + bacterial pathogens was also assessed under simultaneous co-culture of S. Aureus and Pseudomonas aeruginosa. High bacterial loads were required to provide with a statistically significant optical pathogen identification in human serum whereas it was not possible to detect the presence of bacteria at clinically relevant levels in urine.