Because I had several pm's about what exactly cross-resistance is, I'm posting this..
Cross-resistance
To compound the problem of antibiotic resistance, exposure to one antibiotic may help select resistance to others. If an E. coli isolate is resistant to a first-generation cephalosporin (eg, cephalexin), then it will also be resistant to all other cephalosporins of the same generation (eg, cephazolin). Furthermore, if an isolate becomes resistant to a third-generation cephalosporin (eg, cefotaxime), then it will also be resistant to all first- and second-generation cephalosporins.
Cross-resistance also occurs across classes of antibiotics. For example, MRSA strains are resistant to methicillin and its derivatives because of changes to the bacterial receptors for these antibiotics (penicillin-binding proteins); these changed receptors have insufficient activity to result in bacterial death when exposed to the antibiotics. As similar receptors are required by all other β-lactams (including cephalosporins and carbapenems), they too are ineffective against MRSA.
The phenomenon of cross-resistance is one of the principal reasons for recommending narrow-spectrum antibiotics to treat infections whenever possible. If resistance develops, then it will usually be to fewer antibiotics than if a broad-spectrum agent had been used.
Bacteria can also be resistant to many different classes of antibiotics ("multiresistant"). An example is Streptococcus pneumoniae: some isolates are resistant to ampicillin, erythromycin, tetracycline and trimethoprim–sulfamethoxazole20 (case history, Box 2). This not only limits the choice of antibiotic therapy, possibly excluding all oral agents, but also means that the use of any of these antibiotics selects for the multiresistant strain ("coselection"), promoting its persistence.
http://www.mja.com.au/public/issues/177_06_160902/col10836_fm.html
Cell wall deficient bactera cannot be treated with broad spectrum antibiotics. CWD bacteria don't have the receptors mentioned in this excerpt.