- Outer membrane protein changes and efflux pump expression together may confer resistance to ertapenem in Enterobacter cloacae. Szabó, D., Silveira, F., Hujer, A.M., Bonomo, R.A., Hujer, K.M., Marsh, J.W., Bethel, C.R., Doi, Y., Deeley, K., Paterson, D.L. Antimicrob. Agents Chemother. (2006)