infections caused by bacteria resistant to commonly used antibiotics are rapidly increasing in many icus worldwide . whereas multiresistant gram - positive bacteria such as methicillin - resistant staphylococcus aureus and vancomycin - resistant enterococci were considered major healthcare threats at the turn of the century , attention has now shifted towards multiresistant gram - negative bacteria . many bacteria have become resistant to most -lactam antibiotics through production of extended - spectrum -lactamases , rendering carbapenem antibiotics the treatment of choice . more recently , however , outbreaks and increasing infection rates have been reported with gram - negative bacteria , mainly klebsiella pneumoniae , producing both extended - spectrum -lactamases and carbapenemases . only few antibiotics remain active - at least in vitro - against these bacteria . among multi - resistant gram - negative bacteria , acinetobacter species hold a special position because these bacteria are intrinsically resistant to many antibiotics and are capable of rapidly acquiring new resistance mechanisms for the few antibiotics still considered active . patient - to - patient transmission is a prominent mechanism for the emergence of antibiotic - resistant bacteria in hospital settings , especially in icus , and prevention of nosocomial spread of these bacteria is becoming more and more important . from a theoretical perspective , with icu patients confined to their beds , antibiotic - resistant bacteria spread predominantly as a vector - borne disease , with the temporarily contaminated hands of healthcare workers acting as vectors . for this transfer to occur healthcare workers must contact a colonized patient ( or their immediate surroundings ) to pick up the bug , and then touch another noncolonized patient , while still being contaminated , to pass the bug . each of these actions happens with a certain rate , and the product of all these rates is the risk of transmission per unit of time . hand hygiene after and before patient contacts and barrier precautions reduce the likelihood that hands remain contaminated , cohorting of healthcare workers reduces the likelihood that one person goes from a colonized patient to an uncolonized patient , and reducing antibiotic use probably reduces the likelihood that a contact will lead to contamination or colonization . yet there is also one other important parameter in the equation : the likelihood that a first contact will be with a colonized patient . this has been called colonization pressure , first described for vancomycin - resistant enterococci   and later for other bacteria as well [ 3 - 5 ] . in the previous issue of critical care arvaniti and coworkers describe the relevance of colonization pressure as a risk factor to acquire carriage with acinetobacter in a greek icu . in their unit 5.6% of all admitted patients colonization pressure was strongly associated with an increased risk of acquisition , and genotyping demonstrated that all acinetobacter isolates were clonally related . identification of colonization pressure as an important risk factor for acquisition strongly suggests frequent occurrence of cross - transmission ( which is another description of insufficient hygiene ) , especially when this is supported by a clonal relationship of isolates . in the case of acinetobacter , however , as with vancomycin - resistant enterococci , persistently contaminated environmental surfaces may also contribute to acquisition risks , and it is uncertain how this interacts with colonization pressure estimates . the recognition of the importance of colonization pressure also impacts on the methodology of investigating risk factors for acquiring carriage ( and infection ) with antibiotic - resistant bacteria and the evaluation of the effects of infection control interventions . first , in settings where cross - transmission is potentially relevant ( which basically means in virtually all settings with perceived antibiotic resistance problems ) the determination of risk factors for acquisition is suboptimal if colonization pressure ( or a reliable proxy ) has not been included . the pressure increases when a newly admitted patient is already colonized and replaces a noncolonized patient that is discharged or when a noncolonized patient acquires carriage ( and becomes a colonized patient ) . vice versa , pressure reduces if a colonized patient is discharged ( or succumbs ) and is replaced by a noncolonized patient . as icus are typically small ( with 10 to 20 beds ) with rapid patient turnover , the colonization pressure will fluctuate enormously . however , the risk of transmission is not constant and depends on the colonization pressure : the risk is high with high pressure , and the risk is zero if no other colonized patients are present in the icu . this creates dependency in the data and a patient 's status can not be considered independent from the status of other patients . in fact , this dependency is not surprising because it reflects the raison d'tre of infectious diseases . the consequence , however , is that evaluation of an intervention aimed to reduce cross - transmission ( such as hand hygiene improvement , patient isolation , or modulation of antibiotic use ) should not use statistical tests that assume independent observations , such as chisquare tests and student t tests . the recognition of the importance of colonization pressure therefore has major consequences for the clinical epidemiology in icu settings . colonization pressure provides a simple and reliable indication of failing hygiene , but also complicates our approaches to investigate control measures . only well - designed trials combined with appropriate statistical analyses can help us in quantifying the effects of interventions to prevent the spread of multiple resistant bacteria . there are some good samples of such studies that , unfortunately , failed to demonstrate effectiveness of such interventions . nevertheless , we are far better off with such studies than with oversimplified analyses reporting positive findings but neglecting the effects of colonization pressure .
the recognition of colonization pressure as an important risk factor for acquisition of antibiotic - resistant bacteria in the icu , including acinetobacter species , has major consequences for our understanding of risk factor analyses . 
 moreover , the importance of colonization pressure underpins the role of cross - transmission in the dynamics of antibiotic - resistant bacteria in the icu , which has major consequences for the evaluation of the effectiveness of infection control measures .