## 2 Materials and Methods
### Quantitative Assessment of Biofilm Formation  
To evaluate biofilm formation, a crystal violet assay was used. Each well of a sterile 96-well flat-bottom plastic plate was filled with 200 \(\mu\)L of standardized bacterial suspension in brain-heart infusion broth (BHI) (density c.a. 10\({}^{6}\) CFU/mL). Negative control wells contained BHI broth only. _P. aeruginosa_ PAO1 was used as a positive control. The plates  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Strain No.**} & \multirow{2}{*}{**Source of Isolation**} & \multicolumn{6}{c}{**Resistance to Antibiotics**} & \multirow{2}{*}{**MBL**} & \multirow{2}{*}{**Biofilm Formation**} \\ \cline{3-3} \cline{5-8}  & & **PRL** & **CAZ** & **IPM** & **MEM** & **CIP** & **TOB** & \\ \hline
1 & L. resp. tract & S & S & **R** & **R** & S & S & - & **S** \\ \hline
10 & L. resp. tract & S & S & **R** & **R** & S & S & - & **M** \\ \hline
16 & L. resp. tract & S & S & S & S & S & S & - & **S** \\ \hline
21 & L. resp. tract & S & S & **R** & **R** & **R** & **R** & - & **S** \\ \hline
22 & L. resp. tract & S & S & **R** & **R** & S & S & - & **S** \\ \hline
23 & L. resp. tract & S & S & S & S & S & S & - & **W** \\ \hline
36 & L. resp. tract & **R** & **R** & **R** & **R** & **R** & S & **+** & **S** \\ \hline
50 & L. resp. tract & **R** & **R** & **R** & **R** & S & **+** & **S** \\ \hline
52 & L. resp. tract & S & S & S & **R** & S & S & - & **S** \\ \hline
62 & L. resp. tract & **R** & **R** & **R** & **R** & S & **+** & **S** \\ \hline  
& **I** & S & S & - & **S** \\ \hline \hline \end{tabular}  
* L. resp. tract—lower respiratory tract; PRL—piperacillin; CAZ—ctetazidime; IPM—imipenem; MEM—merepememem; CIP—ciprofloxacin; TOB—tobramycin; S—sensitive; I—susceptible, increased exposure; R—resistant; MBL—metaboletalactamases; ()—negative test for MBL; ()—positive test for MBL; S—strong-biofilm producer; M—moderate-biofilm producer; W—weak-biofilm producer.  
\end{table}
Table 1: Characteristics of _P. aeruginosa_ strains using selected phenotypic and genotypic features.  
were closed and incubated aerobically for 24 h at 37 \({}^{\circ}\)C. Then, the contents of each well were aspirated, and each well was washed three times with 250 \(\mu\)L of sterile physiological saline. The plates were shaken in order to remove all non-adherent bacteria. After washing, biofilms were stained for 15 min with 2% solution of crystal violet (200 \(\mu\)L). Then, each well was washed three times with 250 \(\mu\)L of sterile physiological saline once again. After the plates were air-dried, 200 \(\mu\)L of 99% methanol per well was added. The absorbance of each well was measured at 590 nm using an Infinite M200 plate reader (Tecan, Austria GmbH, Grodig, Austria). The interpretation of biofilm formation by tested strains was made according to the criteria of [PERSON] et al. (2004).  
ODc = mean OD of negative control + 3 \(\times\) standard deviation (SD) of negative control; non-biofilm producer (N): OD \(\leq\) ODc; weak-biofilm producer (W): ODc \(<\) OD \(\leq\) 2 \(\times\) ODc; moderate-biofilm producer (M): 2 \(\times\) ODc \(<\) OD \(\leq\) 4 \(\times\) ODc; strong-biofilm producer (S): OD \(>\) 4 \(\times\) ODc.