You are required to list all of your creditors in your bankruptcy petition.  You will want to run a credit report, whether you do that yourself or have your attorney run the report for you.  Everything listed on your credit report should be listed on your petition if there is an amount owed.  However, your credit report is only a starting point.  Not all companies and organizations report to credit bureaus.  Some common examples of places that may not report to credit agencies are some doctors, small businesses, and friends or family members.  You want to make every effort possible to ensure that you have listed all creditors.  In listing your creditors on the petition they receive notice of the filing.
However, if a creditor was missed, there are different options depending on how far your case has progressed.  If you are in a Chapter 7 and your case is still open you can add creditors to your petition.  If this comes late in the process it may hold your case open a bit longer to provide those creditors with notice.  There may be additional fees for adding creditors at this stage.  If you are in Chapter 13 a creditor may file a proof of claim, however, there is a deadline to file claims.  The deadline is set by the court. 
If your case has already been dismissed you cannot add creditors to the petition.  However, as long as the debt was incurred prior to your petition date the debt is still discharged.  In this case you should contact your attorney.  Either you or your attorney can provide a copy of your discharge to the creditor.  It is certainly preferred that all creditors are provided notice through your filing.  In a Chapter 13, if there are creditors that were not initially disclosed to your attorney you monthly plan payment may have to be increased to account for the additional debt.  Notably, you cannot add creditors to your bankruptcy filing where the debt was incurred after the date of your petition in either a Chapter 7 or a Chapter 13 bankruptcy filing.  If you want to list debts incurred after your filing you would have to have your case dismissed and re-file your case. 
If you have any questions, or would like to discuss your case, contact a St. Louis Bankruptcy Attorney today!