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Background Resistance to β-lactams in P. aeruginosa is complex with multiple mechanisms contributing. Since different mechanisms impact different β-lactams to differing degrees, a common dogma is that resistance to one β-lactam does not lead to resistance to others. The purpose of this analysis was to assess the frequency of β-lactam cross-resistance in P. aeruginosa. Methods Unique P. aeruginosa isolated in 2017 at Michigan Medicine were included. Overall, susceptibility (using CLSI breakpoints) and MIC distributions of β-lactams were assessed in all isolates and those with β-lactam resistance. Results 3,836 unique P. aeruginosa isolates were included. Resistance to traditional anti-pseudomonal β-lactams ranged from 15–23%, whereas ceftolozane/tazobactam resistance was 6%. Overall, cross-resistance between β-lactams was common. The table displays select β-lactam MIC distributions for all isolates and in those resistant to ≥1 β-lactam. When resistance of one agent was present susceptibility to other β-lactams was generally &lt;40% with the majority of susceptible isolates having MICs at or near the breakpoint. Ceftolozane/tazobactam provided the best activity in this setting with 65–77% susceptibility. Conclusion Cross-resistance between β-lactams in P. aeruginosa is common. In patients at risk for resistant P. aeruginosa, ceftolozane/tazobactam should be considered for empiric coverage. Disclosures All authors: No reported disclosures.

1600. Susceptibility of β-Lactam-Resistant Pseudomonas aeruginosa to Other β-Lactams: Is There Truly a Lack of Cross-Resistance?